You will need to start by finding a weapon skins. For this guide I will be using CSS Banana. For CSSBanana go here >CSSBanana[css.gamebanana.com]. Find a skin for any weapon you wish to replace.









































If you like the skin you found hit the download button.

A window will popup asking where you would like to save the file to, choose a location of your choice. When it finishs downloading, you will get .zip file, inside would be a Materials, Models, Sound, Script folder.

Highlight all the folders, and drag them into your Garry's mod folder. For me it will be in C:\Programfiles(x86)\Steam\SteamApps\Common\GarrysMod\garrysmod\.

If it asks you to replace any folders hit "yes".

Then after that go into the script folder, and find a file named game_sound_manifest.txt and open it.

It should look like this:

then open your weapons script folder. It should be in C:\Programfiles(x86)\Steam\SteamApps\Common\GarrysMod\garrysmod\scripts\weapons\
then start deleting the lines in the game_sound_manifest.txt that are not in the weapons folder you opened. For example the line "preload_file" "scripts/weapons/ak47.txt"; check the weapons folder you opened, is there a ak47.txt? If not then erase the line, if there is then leave it then do the same for the others. Do Not Delete The Game Sounds or Level Sounds lines. If your gun skin uses custom sounds, download this file and dump it into the scripts folder. {LINK REMOVED}Game Sounds Manifest

After that start Gmod and join a server, or start a singleplayer game, and test out the skins.
*NOTICE*: Some weapons will be left handed, but Gmod does not have any option to switch weapon hands. This will also work for Hl2.
For TTT Players: Go into your \Steam\SteamApps\common\GarrysMod\garrysmod\models\weapons folder and make a new folder called "cstrike"

inside here you will need to COPY the following v_ files or view models from
\Steam\SteamApps\common\GarrysMod\garrysmod\models\weapons or from the model folder from the file you downloaded, depending if you already did the instructions from the very top. You will only need to copy the following files for these weapons if you are installing one these from the list:
scout
awp
ump45
mac10
xm1014
m4a1
usp
glock18
p228
fiveseven
m249
flashbang
grenade
smoke
g3/sg1
ak47
aug
c4
deagle
dual elites
galil
knife
mp5
p90
sg550
sg552
tmp
usp
For Example if I wanted to install a glock skin for TTT I copy these files

Then I move them to the cstrike folder I created and then I change the v to a c

and you are all done! :D

All you have to do is delete the models folder, scripts folder, then go into the materials folder and delete everything in there except for the VGUI and Spawnicons folder in there.