Ganymede is one of the moons of Jupiter. It is believed that it should be one of the bodies taken into consideration when it comes to space colonization. It is far larger than our own moon . It is in fact the largest moon in the Solar System and it has a greater diameter than planet Mercury. However Mercury has more mass than Ganymede. It is possible that Ganymede has liquid water under the surface. If this is correct, it could be terraformed and heated by adding greenhouse gases such as Sulfur Hexafluoride and Nitrogen Trifluoride. Sulfur is found in large quantities on Io, another one of Jupiter's moon. Another advantage is that Ganymede has a strong magnetosphere in addition to Jupiter's, which is believed to be important in holding an atmosphere. However, the 3.5 day night that could become unbearably cold, unless Ganymede's atmosphere is thickened enough to circulate warm air throughout the surface. It's size way is smaller than the of Earth, so Ganymede would not have enough gravity to permanently hold an atmosphere, but it would take thousands of years if not ten's of thousands of years for it to finally lose its atmosphere, and it's magnetosphere and distance from the sun would slow the process. The ice on the surface can be converted into oxygen for the atmosphere, which could be made into Carbon Dioxide. More CO2 could be taken from Venus, which has enough atmosphere, including nitrogen which can also come from Titan, to terraform several worlds.
A terraformed Ganymede would still be quite cold, it would likely be mostly ice with oceans in the warmer areas, though it could be an ocean world if heated sufficiently, but this is unlikely. There would be no land unless a lot of ice was taken from the planet or floating islands could be constructed. The ecology would have very little plant life due to the lack of surface soil, maybe just enough genetically engineered sea and snow algae to recycle oxygen. Ice worms and micro-organisms would feed on that, which would be eaten by birds and fish. This would be the basis of a food chain that could support, whales, seals, penguins and other birds, arctic fox and wolves, polar bears, maybe even Siberian tigers and leopards. Any herbivorous species like caribou and ox would have to be farmed. If life is found in subsurface oceans on the Jovian moons it could also be used in terraforming. Some colonists may be nomadic like the Inuit, hunting for food and travelling by snowmobile or even dog sled.