Where are files downloaded when joining servers in Garry's Mod stored on Linux?

There's a Steam Community guide for that. It's from 2014, but still active.

• Go to .../Steam/steamapps/common/GarrysMod/garrysmod. On Linux it should be inside ~/.steam by default, on Windows C:\Program Files (x86)\Steam (just C:\Program Files\Steam on 32-bit Windows), and on Mac ~/Library/Application Support/Steam.

• Delete the entire cache folder, or just the contents.

• Delete the contents of the following folders at your discretion: downloads, download, Demos, Saves Screenshots, Videos, Dupes. The download folder is things received directly from servers and the downloads folder is Steam Workshop content.