Have a GMod folder that looks like this?
Or just trying to conserve space it doesn't matter, here are three tips to lower the size of your Garry's mod folder.
• Server Files ( Navigation
Every time you join a server addons get stored in this file, I had a grand total of 958 or around 25GB
• Addons ( Navigation
If you have folder addons also known as "legacy" addons, you can clean out what you don't want. If you have the .gma files, then go in game then go into the "addons" section and uninstall/disable what you don't want. You can also do this from the "Steam Workshop" section in your Steam client.
• Screenshots ( Navigation
Get rid of accidental or duped screenshots, I don't suggest completely deleting them as there might be some nostalgic value.