Every mod id ever played has been free.
If you’ve actively played Gmod in a multiplayer setting and you weren’t the server admin: i doubt that. Gmod has a still thriving paid modding community, mostly for extensions for Pointshops, TTT content or other game modifiers. It maybe was free to you though ;)
They surely learnt from that. Now there won’t be an auction house and mail so close together, so players have at least one reason to be in the city. Oh wait, nevermind, they are already selling that for the price of 1,5x AAA game.