The Python source code can be found on Github. Finally, here is a reference to the NeoGAF post explaining the method, and the NeoGAF post explaining the idea behind the optimization of the only free parameter. The quality measure comes from SteamDB and the popularity measure comes from SteamSpy API. Therefore, the score of a game is defined as the product of a quality measure (its Wilson score) and a decreasing function of a popularity measure (its players total forever).

A 'hidden gem' is defined as a high-quality game (hence the 'gem') which only got little attention (hence 'hidden'). This post contains a ranking of Steam games, based on a score intended to favor 'hidden gems'.