Quote: "How do you do this?"
I think I see one line in bold about a retexture and file edit and I think you're asking about just that part. If so, there is a tutorial on Steam that will get you started but I'm not sure how much it applies to the way I did it or how you would prefer to do it.
https://steamcommunity.com/sharedfiles/filedetails/?id=408830830
The tutorial was done with Gimp and I guess other applications. I personally use Photoshop with Nvidia's Normal and DDS plugins. The retexture was the easy part. Editing the associated files for weapon behavior were the harder part for me. I made a lot of mistakes and had to redo several things the first time around editing an existing weapon for GG. Make sure you've had plenty of sleep before you try.
Before anything else, make a copy of all necessary files and work on the copies. There are other ways but that is always your best bet.
If you want my honest suggestion, try some really simple texture changes first. Add some stripes or other random changes to see how it looks with the colors you want to use.
(Don't try to do a full, beautiful retexture on your first attempt unless you are already comfortable with your image editing skills. It may be a waste of time until you get the hang of the various image mapping involved. It's not as hard as it may sound.) Then you'll have a better idea of how much the Specular maps and such need to be changed to go with your chosen color scheme and even your map's chosen visual settings.
I hope this is some kind of answer for you to understand and use.
I've attached a picture that has one of my changed pistols on the left and the original on the right. Aside from base colors and actual weapon behavior, the biggest change was the specular map. The original was too shiny for the map and theme I was working on at that time, including the skin on the arms. The top image is a side by side of the actual pick-up item, scaled way up on the map, with changes not as obvious.
I'm planning to do another remake soon. I could try and do a video tutorial at the same time, but I can't guarantee exactly how soon that will be. Anything that needs a voice is hard for me because I live and work in a very busy, noisy home.
I'll add an avatar and a real signature someday. I think...