You cannot use Gmax for editing existing aircraft. Use a standard graphics editor like PSP, Photoshop or one of the free graphics editors to edit the textures. Pixia is a good freeware graphics editor. It's posted in the SimV design sections. Almost any graphics editor/paint program will do to start with. M$ Paint can be used if you have nothing better.
You willl most likely need DXTBmp here. http://www.mnwright.btinternet.co.uk/index.htm

This tute might also help. http://www.gjsmith.com/Tutorial/Tutorial.htm

I just now got Pixie.  I dont even see textures in the aircraft files?  What's going on?