There are literally millions of free maps, mods, add-ons, and even free full playable games made in Garry’s Mod that you can enjoy. If you’re creating a game (or even a feature film!) and you need assets, you can just go check the Workshop and see if there are models, textures, or scripts that someone else already made so you can get the work done. The Steam Workshop is really what sells this game, honestly. Garry's Mod, also known as “GMod”, is an amazing indie game that lets you create any kind of games you like and share them with other users – just keep in mind that the latest version is paid but you can always download and share mods for the game for free from the Steam Workshop.
Garry's Mod is hands down one of the best sandbox indie games of all time.