These are additional packages with textures and other content to solve modification problems in Garry's Mod.
If you have problems with the "missing textures" of mods or their incorrect operation, then you can insert these ready-made add-on packs into the game files by: *steam-location* > steamapps > common > GarrysMod > garrysmod > addons
These packs contain content from other Source games whose textures or models modders often use in their Gmod's add-ons