Setting Gaia up to use your own textures is generally a fairly simple and fast process.
- Create a duplicate of your Gaia Resources file.
- Create a new terrain using the new resources file with Gaia.
- Update the textures, grasses, trees in the new terrain to whatever you want (i.e. physically add them / swap them in your terrain object using standard unity workflow)
- Apply these back into your new resources file by selecting it and clicking 'Update From Terrain'.
- Update the spawn rules in your resources file if you need to. If you don't the old ones will be retained.
- All new terrains created with the new resources file will retain the new settings.
For video tutorials on how to do this and manage resources in general please check :
Terrain Texturing : http://www.procedural-worlds.com/gaia/tutorials/spawners-unity-terrain-textures/
There are bunch of video's on this and many other topics available here: http://www.procedural-worlds.com/gaia/?section=tutorials