Since gMAX only exports to proprietry formats its all pretty much academic. As I understand it, using gMAX for anything other than lisenced gamepacks is illegal, AFAIK. Think about it, based on what you just said, why would Discreet give away a product that does everything that their flagship product does, but for free?
There have been quite a few threads on this....search the 3D forum for "gmax". I think you used to be able to use the Flight Simulator gamepack to export static meshes in .X format, but I don't think that works anymore with the later version.
Its a little like the Personal Learning Edition of Maya 4.5 which also has a restriction on the file formats it saves to, as well as watermarks on any renders made. gMAX is a "learning edition".....good for learning the software, but pretty much useless for anything else. It works for Halo because it is made by Microsoft, which has a lisence agreement with Discreet to provide gMAX supported gamepacks for its games.
Basically, I don't have a definitive answer for you, but I'm pretty sure you'd be wasting your time trying to use gMAX to create levels for DB-based games.
Tron Software