Hi and welcome to Simmers Paint Shop.
Firstly, I don't have that particular aircraft but I've seen this before and I think it's down to the actual aircraft model not supporting reflective textures. This option has to be enabled during the modeling stage in GMax (make alpha reflective), otherwise the alpha channel will make the model transparent like we see here.
Your screenshot in DTXBmp shows that the alpha channel is there, and the plane will be more transparent in areas where the alpha channel is darker color. To solve this, you can click the little button under the alpha channel preview that gives a blank new alpha. (Be CERTAIN to backup the repaint before saving over it!)
Lastly, you'll have to figure out if this aircraft model actually does support reflections. Does the default livery that came with the plane show reflections? If it does, then there may be a settings issue. Post back on this and I'll look into if further if so.