In the Mod
Gallifrey is covered in Gallifrey Grass, with rare spots of Gallifrey Sand (An ingredient in the Elixir of Life). Underneath only Gallifrey Dirt can be found. On the surface, trees made of Oak Logs and white Gallifrey Leaves spawn. These trees cannot be grown with saplings.
On Gallifrey, Time Lords and Chancellery Guards spawn naturally.
In Update 53 - the Citadel could be generated, but this would only happen very very rarely, resulting into only a handful of players in the community finding it. The developers have confirmed they are trying to find a good way to generate the citadel so it can be found.
In the 1.7.10 Legacy version, near X: 380 and Z: 184, a large glass-domed city known as the Gallifrey Citadel spawned.