Teabone a _G map is a glow map, it's the same as our _I illumination map, I think there's only about six or eight actual formats but you will see a lot more because different developers use different standards for labelling them.
The cube map in gameguru is an environmental cube map, it's basically a flattened cube interior texture, it's the same as Skyrims _E map. They're used for fake reflections and shadows on weapons and armour etc. So that it doesn't have to constantly do a full render for reflection on the object.
Other maps I don't think we have yet are:
_B blur
_H greyscale/parallax height map, this works with normal maps to make grooves etc deeper and ridges more prominent, sharper.
_AO ambient occlusion for adding AO shadows, we just bake into a diffuse instead here.
_EM emissive map, similar to glow maps but deals with real time reflections i think.
