Hi,
I can only speak about what's technically possible in the current alpha, as I have no idea how the GM mode actually works and how it changes the game engine.
1. The game uses standard DDS textures, so they can be imported quite easily. As there is no GR2 import pipeline available officially (I assume it'll be available when the editor is released) a 3rd party tool is needed for importing meshes; see:
http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=572322http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=564303#Post5643032. Same as the above; no official tooling available yet, unofficial tools available.
3. I'm not aware of any "full" modding guides.
4. You mean something like a heightmap?
5.
It was not possible in D:OS 1, D:OS 2 status is unknown so far, because the version of the game with GM mode is not publicly available yet.
Edit: Technically it could be possible, however I'm not sure if anyone ever tried something like that.