Hey, Garry's Mod addon system has changed a bit from the start.
Some old add-ons may tell you to paste your add-ons to /garrysmod/ folder, but by doing this you merge and conflict the add-ons files with the game base files.
This leads to conflicting gameplay at times and there's almost no way to uninstall add-ons, unless by picking each file by hand.
For this manner the developer added a specific folder called "addons" under /garrysmod/ where you can keep each addon it's own folder.
And this also eases up the removing of addons. Which can be done by just deleting the folder of the add-on.
Here is an example structure from the folder view
~/.cxgames/default/drive_c/Program Files/Steam/steamapps/username/garrysmod$ tree garrysmod/addons/Stacker\ STool/
garrysmod/addons/Stacker STool/
|-- info.txt
`-- lua
`-- weapons
`-- gmod_tool
`-- stools
`-- ol_stacker.lua