With exportation system we have development it is possible to export from MSTS files to Trainz files without using Gmax. By the way, a lot of the models we export to Trainz were former made with Train Simulator Modeller!.
Really our system it is not a easy way, but it works fine. Theoretically, any MSTS model (rolling stock and objets) can be export to Trainz without problem.
Almost all the Trainz asset you can download at TrenSim were former MSTS asset! Of course, we have always the former MSTS creator's permissions.
Javier.-