As the name implies, Garry's Mod was originally a Source Engine mod that let the user muck around with that engine's physics, models and props. The mod released in 2004, before spawning a standalone product on Steam in 2006. Now, in 2021, Garry's Mod has sold an astounding 20 million copies.
That's pretty impressive for a game that has no goals, no narrative, no incentives: You just boot it up and stuff around, though players have managed to create some pretty bonkers and brilliant maps, mods and modes in it. According to Garry Newman in our 2019 interview, the game's roleplay potentials are one of the big draws for players. "The roleplay gamemodes are still a huge thing," he said, "but that in itself can encompass a thousand other sub gamemodes."
Despite a kinda-sequel being on the horizon, Garry's Mod sales seem to be accelerating rather than declining: the game hit its 10 million sales milestone in 2016 after ten years on sale. It's taken an additional five years to add another 10 million to that figure. Sadly though, Newman doesn't get a dollar for every copy sold.
As for that kinda-sequel, it's called s&box and it's coming along nicely. Plans to use Unreal Engine 4 have been abandoned in favour of Half Life: Alyx engine Source 2.