You should show the folder where the materials are stored for the model, if you don't know it's compiled in the 'addons' folder for Gmod (if you don't know search up gmod legacy addons) and it will contain the model you have compiled, and go in the materials folder and and go in the models folder in there again and you should find the materials
It should contain the 'vtf' files which are the textures, and 'vmt' which are materials, if you don't find the vtf files VTFCmd failed to convert them, which I wouldn't be sure why but if you imported textures tell me what file format they are