I looked into this a bit over the last few days, and I'm still not sure.
The .gm format contains preprocessed BSP information that would have to be built during the file-read of an .fbx, which would be quite slow. I am still investigating the possibility and I would need to test the slowness. If I successfully replicate that information, yet it is slow to build, I might just create a converter instead, that would be used to change .fbx to .gm for the game resources. I'm also still unclear on some of the details involved, so I cannot yet say whether this will ever happen, but I'm looking into the idea at the moment.
So that is neither a "yes," nor a "no" answer for now.