No, you don't need Gmax for that. Gmax is used to build the model and to map textures to it. What you're looking to do is what we call 'reskinning' and there are lots of threads around about reskinning and I believe there's some tutorials around on the subject. Basically, what you'll need is a photoediting program to edit the textures that are applied to the model. These texture files will generally have either a .tga, .bmp or .jpg extension. I use photoshop for textures, but there's lots of programs out there. I believe that a lot of folks use GIMP, which I believe is a free program/download.
You can clone the object that you're wanting to reskin in CM and then go to work on the cloned object to reskin it. Word of warning though, in most instances, you will need the permission of the original creator to upload or redistribute a reskinned object.
Mike
