Garry Newman, the game's creator and namesake, cites a number of reasons for its success: the ever-increasing popularity of Steam, user-generated content, additions from other Valve games like Team Fortress 2, and the modern prevalence of video game footage on YouTube. That last one is crucial, as Garry's Mod thrives on its weirdness. The game itself allows players to place any Source Engine object into the world to manipulate as they see fit, and as the above screenshot demonstrates, the best results are often bafflingly weird.
Surprisingly, Newman believes that Minecraft – a similarly open, wildly popular sandbox game – is more of an aid than detriment to sales of Garry's Mod. "You'd think Minecraft would kill GMod's popularity ... but it's kind of the opposite," Newman told Rock Paper Shotgun. "People play Minecraft to death – they love it ... so they go out looking for other games that are a bit like it. Minecraft has attracted a lot of people that aren't really PC gamers too. They play the game on the 360 or the iPad and everyone tells them that the PC version is better."
While current monetary figures don't exist, Newman revealed in March that at the time Garry's Mod had generated $22 million in sales so far – quite a haul for a $10 Steam title that's never seen a proper retail release.