The best thing about GURPS is the hundreds of world books all using the same core rules. With so many resources it seems a waste to play in one specific genre. The campaign is based on the world of Kel, which exists on the back of a large turtle swimming through space.
Yeah, a turtle. Clearly this leads to a few problems... these will be covered later. The turtle is just a good base of operations with access to many other worlds and dimensions. Aside from the portals, there are legends of visitors from outer space. Many wonder what would happen if their turtle landed on a larger planet
This is a good place to begin a campaign if you want to create the majority of the party as fantasy characters and still travel to different dimensions. Characters that are from other eras are quite feasible. A gunslinger could find himself here through a portal with a droid who landed on the turtle in an escape pod from across the universe.
Sometimes the turtle follows an orbit - but often he will float around apparently aimlessly. So days and nights are not always on schedule. Sometimes nights may last a week while the turtle is swimming away from stars. The climate on the plates is not seasonal, however, and will remain as noted under geography even in cold space. Needless to say, navigation by the stars is near impossible.
There are legends of continents floating through space on other things, most notably elephants and stingrays. And large balls of rock.
Most of society is in the early Renassiance. Cities further from the center are less civilized, dropping to TL 3. There are even quite a few pockets of primitive society and vast lands owned only by the wild.
The turtle's shell houses the majority of residents. Travel on the shell is usually pretty easy. Riverboat is the easiest since each scute is separated by a river. What is a scute? A scute is a plate on a turtle's shell. There are 13 scutes (not counting the outer ring). The main scutes are approximately 100 miles across. The outer ring of scutes is all covered with water. Expeditions onto the tail, fins, or head would be extremely difficult.
The scutes will not be detailed individually in this article. The ones near the front are cold and mountainous. The ones near the rear are hot and wet. Two of the last 3 are totally underwater. In-between is relative heat and terrain, and where most people prosper.
The soil on the shell is accumulated dirt and rubble. It is fairly thick and no one has heard of burrowing through to reach a shell. People have seen the edge. And the ones that fall off are gone forever. See the GURPS Space rules for suffocating and decompressing outside the shell.