Not bad for a physics sandbox with no subject or goal.
As the name suggests, Garry's Mod was originally a Source Engine mod that allowed the user to tinker with the physics, models, and hardware of this engine. The mod was released in 2006 before releasing a standalone product on Steam in 2004. Now, in 2021, Garry's Mod has sold a staggering 20 million copies.
This is pretty impressive for a game that has no goals, no narrative, no incentives: While players have managed to create beautiful craziness and brilliant maps, mods and mods, you just launch it and play around.
Despite some sort of sequel on the horizon, Garry's Mod sales seem to be accelerating rather than declining: the game hit the 2016 million sales milestone in 10, after ten years of sales. It took another five years to add another 10 million to that figure.