When you want to install addons, first check if the addon has an “info.txt” in it.
Meaning, if you enter a folder, say for example, “AK47 Swep” and there is an “info.txt” in it, you would put the folder “AK47 Swep” in the [sub]Steam\steamapps<username>\garrysmod\garrysmod[/sub]addons folder.
If the package you downloaded does not have an “info.txt” but folders like “materials, models, sound” etc. etc. you would put all 3 folders into [sub]Steam\steamapps<username>\garrysmod[/sub]garrysmod\ and let it overwrite anything.
If the package you have is something like “AK48 SWEP” and inside that folder theres “AK48 SWEP,” enter the folder again to check for an “info.txt”. In the event where there is a folder in a folder with the info.txt, drag the folder with the info.txt into addons and not the first folder.