This needs to go into a file named info.txt in the folder for your addon. This information is primarily for addon writers. This is the structure of an addon folder. This is the most common problem when installing wiremod and it won't work. REMEMBER, if the info.txt doesn't exist in your addon folder, it wont get loaded when garrysmod loads. This info.txt contains information about the addon itself, read below how to set it up correctly.

For garrysmod to take the stuff inside your folder, a info.txt needs to be located in the FIRST folder, the folder at 'username/garrysmod/garrysmod/addons/MyAddon/'. However, if the addon ain't built correctly, nothing will happen at all. You can just remove the folder from your addons directory, and now, it doesn't exist anymore, so you don't need to search through every folder in 'garrysmod/' directory for files. This is nothing noticeable, as you can't see the files in those folders. When garrysmod is then started, every file in every folder in addons folder will be sent to the 'username/garrysmod/garrysmod/' path instead. An Addon is a folder with files in it that goes in the 'username/garrysmod/garrysmod/addons' folder.

How to make gmod addons