Leaderboards: Example of Implementation - GOG Developer Docs

Documentation for GOG Game Developers

Sales Leaderboard

How To Add Online Leaderboards In GameMaker

Stats and Achievements: Example of Implementation - GOG Developer Docs

Leaderboards - GOG Developer Docs

Leaderboards Screen - GOG Developer Docs

GOG Galaxy 2.0 Closed Beta Review - One place for all your games

20 Best Game Development Books of All Time - BookAuthority

GOG Galaxy adds Epic Games Store integration

Identity Provider Management Epic Online Services Developer

You don't need to outsource porting

GOG GALAXY 2.0 - All your games and friends in one place.