Well, export the gear's texture into formats like tga or png using VTFEdit, then just edit them with Photoshop or any image editing software like that, after you done, just import them back and save them using VTFEdit.

Also, you need to make a vmt as well if you want to make it for Gmod.

I'm looking for a way to make my own custom texture to my OC. anyone know a tutorial or tell me what programs to use?

Well maybe eventually. not now since I got some priorities first, pardon.

To @NeoLin:
Can you port this to the Gmod workshop?: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2378339334