What'll you need to make your own skin(s): The original .tga files supplied by roast
WinRAR (For the original .tga files)
Any image editing software VTFEdit
Once you've got your skin cut and clipping masked. Save it as a .png. We'll use a simple Red Tiger camo from MW2 as our example skin.
Open up VTFEdit and go to File > Import and select your skin. You'll be greeted with a screen like this
Within the spoiler below should be all the information you need on picking the right settings for the best outcome
Once you have your desired settings save it as a .VTF file. Make sure you call it the same as whatever the weapon part you chose was called. For eg. we change the receiver from the ak folder.
Go to your garrysmod addons folder, commonly found at C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\addons
Find the desired addon, for us, we're using the AK-47 from the M9K assault rifles pack.
Open up another instance of file explorer and head to C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\bin here you'll find a file called gmad
All you need to do is drag the addon file on top of gmad.exe and it'll create a folder with everything needed to see your skin in game.
This should open a command prompt just give it a few seconds and it'll be done.
If you scroll back up to the top of your addons you should now see a folder.
Create a backup of this folder so you can access the original materials, or if you can't be bothered you could always reinstall the addon after you're finished.
You're going to want to open it up to here, C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\addons\m9k_assault_rifles_128089118\materials\models\weapons\v_models
This'll give you access to all the weapons in the pack so if you want to do it on one of the other weapons you can. For us, we're going to open up the folder called "thanez_ak". Once open replace the existing file(or files depending on what and how many parts you altered) with your textured skin. Open up or tab back into gmod and your skin should be on your weapon.
Obviously, there's much more you can do and each weapon has multiple parts, but that's for you to figure out (because I can't get my tiny brain around it yet) You may want to be careful because certain parts will cause your skin to be flipped.