Basics of Downloading/Installing Addons:
When you download a file, you may either start with a folder, a zip file (.zip), a winrar archive (.rar), or some other form of file (.7zip, etc.) For opening all types of files that are not just a folder alone, I recommend 7-Zip, (http://www.7-zip.org/download.html), it is a free program that does everything a purchased program can do.
Once you open your file, you should have a folder (or you should start with a folder) with something similar to the addon you downloaded. Open the folder and take a look at the contents, if there is an “info.txt” file in the folder, then the entire folder should go in your garrysmod/addons directory. (C:/Program Files/Steam/Steamapps/<Steam Name>/Garrysmod/Garrysmod/Addons), you should then see the folder you added with the info.txt inside.
If the file does NOT have a “info.txt” file, your going to add the files straight to your garrysmod directory. (C:/Program Files/Steam/Steamapps/<Steam Name>/Garrysmod/Garrysmod). Your going to want to look for files that say “Lua, Models, Materials, Data, Sounds, Scripts, Maps, Gamemodes”, etc. Move those files directly into your Garrysmod folder and it should ask you if you want to overwrite or combine the folders, say “Yes” to all of them.
Added Note for Addons: The more addons you have, the longer loading times will become when you first start Garry’s Mod as well as when you host a server, you can reduce this by moving everything within the addons folder into your garrysmod folder, but do this at your own risk, as for sometimes you need to remove addons, and doing so in this manner is troubling.
Black and Purple Textures:
When you have Black/Purple textures, it means you are missing one or more texture files for that particular model, texture files are most likely in the “Materials” folder in your garrysmod directory.
Note: When downloading models, take attention to any “Required:” sentences in the release post. Most of the time custom models/addons will come with everything you need, but sometimes they require a certain game to be downloaded and installed. If you do not have the required game downloaded and installed then you will get the back/purple texture problem.
If you have any further questions in Garry’s Mod you are more than welcome to either PM me on the forums or add me to steam (Search: Kinglingchow@aol.com). I hope I was able to offer some assistance.
Forgot one thing. About multiplayer servers, if either you or your friend host a server, whichever one is hosting needs to make sure that you have the right ports forwarded on your router. Check out http://www.portforward.com to see how to forward the ports for your router, and also check out the “Required Ports for Steam” section on Steampowered.com (https://support.steampowered.com/kb_article.php?ref=8571-GLVN-8711)
Final note on server hosting. Most of the time you will not be able to right click on your friends name and click on “Join Game”. Most likely you will have to figure out what the server name is and look through the server list for the server you are wanting to connect to. (It sucks, but have your friends server in your favorites tab for the next time you two play together. [If he changes the server name you will have to search for it again in the server list])