Garry’s mod uses Valve assets and is published by Valve. Of particular note, it has half life assets in it.

The skibidi toilet series was made with Source Filmmaker, a video editing software published by Valve, which allows people to use the Source engine, the game engine that half-life 2 used. As SFM was made by Valve, they allowed a bunch of half-life assets to be free in SFM. the original toilet head is an asset from Half-life 2, Male_07, which Garry’s Mod has access to, given it is a valve release and can use valve models.

They C&D’d Valve for using Valve assets in a Valve game.