This is a relatively low-fantasy game setting, but it has sci-fi elements as well. The Kingdom of Nool has a limited concept of Astronomy. They watch the stars and study them, but they know about as much as Earth-based Europeans knew circa 1200 AD. This is the case for most societies in the game universe.
But, it gets more interesting than that. The various societies of humans which make up the game universe are spread amongst billions of planets connected by a system of ancient portals of unknown provenance. These aren't usually hidden or unknown. Many cities and kingdoms are built where multiple portals meet. The way societies spread is not determined by the distance of planets in space, but by the network of portals (which is utterly arbitrary and seemingly infinite).
When travellers use these portals and find that the stars are totally different on the other side, this upsets any notion of consistent astronomy! Curiously, all of the connected worlds are Earth-like in gravity, climates, and atmosphere (although they vary in the time, length, and intensity of seasons such that someone traveling by portal often must plan for multiple seasonal climates). The prevailing consensus among learned people regarding cosmology and the structure of the universe is, consequently, not even close to the physical reality; and there are as many theories as there are religions. In some places, however, this wealth of reference points has led to relatively low-tech societies which have a very accurate concept of astronomy (Nool is not one such place).
Usually the portals are considered something like terrain features which have "always been there". They usually take the form of doorways (sometimes large gates or even colossal, martial-looking archways) which look straight out into the place they lead to, as if looking through a literal open door. But, some are more hidden. Some don't look like portals at all. There does not seem to be a hard-and-fast rule to the form these portals take, and the fact that most of them follow a similar design can only cause learned people to guess as to why.
Whatever form they take, these portals are invariably more ancient than the cultures which use them and they are taken for granted by travelers wherever they are oft-used. Travel through them is usually watched, if not regulated, by whatever political force controls the territory they sit on (as they make for natural economic and military choke-points in many cases).
In areas of the network where they are densely grouped and oft-used, the connected locales can even form a single modest city spread across multiple galaxies (populated by people who don't realize they live on different planets). In other places, the portals are long-lost and hidden in deep wilderness. In others still they are regarded as dangerous fonts of invasion (by inhabitants on either side of one, or mutually) and physically sealed for generations at a time. No unified culture, religion, or state unifies this mass of humanity; most societies have a scope of cultural reference no larger than a medieval Earthling kingdom.
High TL civilizations do exist, and some of them even have space craft and a solid understanding of the physical universe (including advanced Astronomy) but they are so far from Nool as to be mythical or unheard of, and do not have any bearing on the way learned members of Nool society view the universe. The portal network is so large that even a TL12 civilization at the height of intergalactic conquest would struggle to militarily secure even a fraction of it (and in some isolated regions -- which still amount to entire galaxies -- this is the case), even though the vast majority of it is peopled by TL3 civilizations who are barely aware of each other. All of this begs questions about how the portals came to be, in a way that's so obvious that most people stopped asking thousands of years before they had the science to realize how bizarre the portals are (in those societies advanced enough to practice advanced science in any recognizable form).
While they are taken for granted as features of the terrain across most of the game universe, it is obvious to all that they are the products of architecture and not natural features of the landscape. In less advanced societies they are considered the result of ancient gods, in almost all cases. In lower TL societies with an advanced understanding of Magic, they are usually assumed to be the products of powerful ancient sorcery. TL12 civilizations are capable of reverse-engineering the portals and building their own, but could never hope to explore a fraction of the portal network even over a span of billions of years. The best scientists in the most advanced societies in the known universe can only shrug at hard scientific evidence of a portal network that should have taken longer to build by far than the universe has existed, according to the best theories of cosmology (most resembling the "Big Bang" theory we are familiar with here on Earth).
While some Psis can teleport, and some Mages can do the same, and plenty of societies have an advanced understanding of the supernatural, no society below TL12 is capable of figuring out anything significant about how the portals function or of creating transportation technology (magical, psionic, or otherwise) which can reach other planets on the physical plane of existence instantaneously. Many Mages and Psis are well aware of other planes of existence. It is commonly understood among learned Mages and Psis that the portal network connects locations on the physical plane; not to alternate dimensions or separate planes of existence (in magically and/or psionically advanced societies which allow them to organize higher learning, anyway).
Sometimes portals will shut down or start up of their own accord. Sometimes they will suddenly point somewhere else. Sometimes new ones are unearthed or discovered. All of this creates a very fluid picture of humanity spread out in a way which frequently splits and merges with chunks of itself, leading to an infinite variety of isolated civilizations. Usually portals will remain stable and fixed connections between two points in space for thousands of years at a time.
WARNING: Spoilers for this and other fiction below.
[Note: sophisticated players will recognize that this mechanic has been used very effectively by many sci-fi settings in order to enable an infinite variety of different but connected societies. The plot twist from Mass Effect (where the "big bad" is in control of the portal system all along in a way that ultimately defines their entire existence) would be good here too, but I will strive to not be so predictable in light of the massive popularity and success of that series (anyone likely to play this will be familiar with the potential twist).
The way portals are used in Stargate is a little closer to how these will be used (the system, though a precursor remnant, is not really under any faction's total control), but unlike those portals these are not devices which can map to any number of other connected devices if you only have the right code. They are instead only ever mapped to one other portal at a time, for centuries on end, before re-routing their payloads without warning to some other location for several more centuries, and repeat. This is mechanically important, because it results in the repeated isolation, merging, rise, and collapse of societies which could be any tech level and not be a part of any centralized empire or state (anymore?), merely as a result of these things having been "left on" before recorded history and without any intervention on part of the societies which currently use them (not even a dial code -- just walk right through).
That the portals take centuries to change their routing destinations means that societies might grow to depend on them as they are located in ideal positions, economically and geographically, for many societies (or even relatively modest cities, in some cases) to span several nodes on the network... only to become suddenly fragmented when the portals shift their routing at the height of a golden age, or in the middle of a crisis (any time is a crisis when half of your country suddenly disappears and is replaced with a new land populated by strangers, if it is populated by anyone at all). Thus, the simple mechanical rhythm of the network is responsible for churning the infinite mass of humanity like a spoon through cultural and technological oatmeal. This results in the situation of TL3 medieval societies being the norm with isolated societies of every TL being able to arise on the network somewhere, on some given isolated leaf node. A space-faring TL12 society spanning an entire galaxy might be relatively isolated on the node and be mostly unaware of the untold billions of TL3 societies which are just far enough away from their TL12 neighbors (by the idiosyncracies of the portal network) to be safe from aggressive expansion or colonization. Even that TL12 society will be baffled by the scope of the portal network, in the event that they try to use it for conquest (which some do).
Nevertheless, I do not promise not to involve aliens at some point (I certainly will, eventually, along with some fantasy races and beings from other planes of existence); but even when I do they will be a small mote in a much larger universe.
These portals serve as a "Precursor MacGuffin" and as a contrived way to people a broad universe, but I will not make it so easy on the players as to re-use the plot device of having the "big bad" invade the game world through the portals when introduced to the campaign. The portals will of course have esoteric mysteries about them to uncover which deeply affect the players' evolving understanding of the setting, but the "big bad" won't be invading through them. The portals are regularly used for military invasions by some of those who have access to them, but that's business-as-usual and the "big bad" of the whole campaign is not going to suddenly turn out to have been in charge of the portal network. If I ever decide to clarify their origin then I will choose some other trope. I say this here to spare the players from anticipating a major plot twist that is a bit too predictable in 2023. That said, players can rest assured that the campaign will feature a great many predictable tropes because that just can't be helped. I'll do my best, in any case.
Quite frankly, in order to use most of the content in the GURPS manual in a single setting, you must (in addition to allowing for various shades of the supernatural) either have a portal network, interdimensional travel, and/or space travel. Most settings (for example the "GURPS Infinite Worlds" stock setting, which focuses primarily on alternate realities) will likely include all of those options, as this one also does. The choice of a portal network was primarily made so that I can play with the idea of humanity spreading out infinitely but not smoothly or homogeneously, while being able to maintain a primarily fantasy feel except when/if the campaign reaches beyond the initial starting region. So, although a bit of a kitchen-sink approach, this is a TL3 universe that inherently has the plot-capacity to be up-scaled to include any of the tech levels and powers I want to use for a given campaign. Perhaps the next one will take place on a TL8 world, and the players discover a hidden portal leading back to the TL3 world they explored in the first campaign with their first characters; except this time they are the Outlanders, and their relationship to the locale is entirely different; perhaps hundreds of years have passed in the game setting since the first campaign and the players get to enjoy the butterfly effect their previous characters unleashed over time.
As the "big bad" will involve something resembling a "zombie apocalypse" (it is not a spoiler to say that much -- the players should know this before it begins!), I also want to explore simulating its spread throughout the game universe. I want to give the players opportunities to use the graph of the portal network (and more generally the networks of roads, rivers, and terrain which make up the maps of the affected regions) against the "big bad" when it shows up.]