Erez. It's not clear from your question if your texture problem is in Gmax or the sim. To narrow it down try assigning some existing textures to your model. If that works the problem is the textures.

I can't help with Gmax but if they don't display in the sim make sure the textures are in the correct size & format. As a repainter you already know about FS texture formats. Aircraft textures are commonly square, usually 256 x 256, 512 x 512 or 1024 x 1024. They should be either 8 bit BMP or one of the special 16 bit Extended BMP texture formats. Also, use texture file names without any spaces. This is a common cause of textures not displaying.

Apart from the basics I have no knowledge of Gmax. Being used to the traditional FS design tools I find it overcomplicated & difficult to understand. I did try some simple test pieces before FSDS2 was released with some success. I suspect your problem lies in the UVW mapping. The parts have to be configured to accept texturing before they will display.

PS. I'm a great believer in using test pieces to experiment with any new process. It's much easier & quicker to find out how it's done by following a tute based on a simple object. It always surprises me how many would-be designers leave it until their project is complete before learning the essentials of animation & texturing or exporting it to the sim.