system
A chat on subreddit /r/me_irl
human
Holy shit that can't be right... Can it? 

Gary's Mod was released November 29, 2006. This was 5,901 days ago, or approximately 141,624 hours. They would have to have spent almost 16 hours a day every day since the release to accomplish this.
gpt
They apparently ran a rather popular RP Server, so I'd wager most of their time was relegated largely there either in gameplay or some form of administration. It's certainly an extreme example, but any time this discussion comes up on reddit, it's clear my meager 3,166 hours is petty. 

The average hours played for the game is 162 and the top 10% average 1,515 hours. 3 billion hours. For reference, about 3 billion hours ago, mankind invented *rope*.