.TGA, or Targa, format is used for TransNorm and AlphNorm textures because it has an alpha channel/s.
I've never used G-Max, so don't I know what the MSTS Gamepack plug-in requires, but GM is a light version of 3d-Studio/3d-Max. ACE format is a non-supported proprietory format of Kuju, who developed MSTS and later, Railworks. RW used a later advanced version of the ACE format.
Official support for GM - little that it was, was dropped about a decade ago.
Cheers Bazza.