If you are trying to edit bitmap.tag files you need to first export them into a usable format which gimp or photoshop can read.
Preferably .tif since halo will only accept that format back on import.
Btw it would help if you could give us more detailed information with regards to what you are trying to achieve.
I.e
Are you just trying to edit a texture?
Are you wanting to apply a texture to level mesh in gmax?
Here is something I picked out at random hopefully it may be of some use.
https://m.youtube.com/watch?v=tLJqkka3TjY https://m.youtube.com/watch?v=pkxdRdlSx2Q