I never used any gtl.tex file to do skins... Maybe you mean "how to open xxxtex.gtl files ?"
Double click on any xxx.gtl file and tell windows to open it with geditor, then you will be able to import/extract dds in order to skin them
hold on i think I am getting it. My geditor was not on my c drive so i copied it to my program files folder and directed my tex.gtl files to it ,now I can open in .dds with paint net and paint til my hearts content. thanks loads