Steam Spy is a tool that scrapes data from Valve's own publicly available information. Using this information Sergey Galyonkin aggregated data and estimates that Steam was responsible for 15 percent of global PC game sales in 2015.
Excluding all free-to-play games Galyonkin reckons it amounts to up to $3.5 billion dollars. Over 3,000 new games have been added to the Steam store in 2015. Steam Spy, created by Galyonkin, uses Steam's own data to extrapolate sales and ownership information for games (albeit with a three-day delay). Grand Theft Auto V and Fallout 4 lead the way with $161 million and $123 million in sales. For total games sold they also maintained the number one and two slot, with the third place going to Ark: Survival Evolved and fourth to Rocket League. H1Z1 landed on the fifth spot.