Gmax is a cut down version of Max, Maya ple is packaged with UT2003, amongst others and there is a growing mod community using it and Maya.

The real issue is compatability with the game engine you are working with but there are plugins and things about the place.

In terms of free / cheap alternatives to the full versions of things I would rather work in Maya PLE or indeed even Milkshape than Gmax, but I have never been a Max fan. I don't find mod making in Maya to be a problem in terms of useability, my sub standard skills notwithstanding .