SteGOG от Ugzuzg

Adds an indicator on gog.com, which will let you know if you own a game on Steam.

You will need to get your steamID64 (to obtain enter your username here https://steamid.io/ or use any other convinient method). Once you have it you can paste it either on the addon's settings page or on gog.com homepage into an ugly input at the top-left of your screen (should appear after you install the extension).