No revenue estimate for GOG — its rating/review counts don't calibrate to sales the way Steam's do. The Steam version is estimated at $11.1M — a different platform, not this one.

Undertale

In UNDERTALE you play as a human child who falls underground into the world of monsters. Now you must find your way out... or stay trapped forever. Welcome to UNDERTALE, an RPG game where you don't have to kill anyone. Each enemy can be "defeated" nonviolently. Dance with a slime. Pet a dog. Whisper your favorite secret to a knight. Or, ignore this choice and rain destruction upon your foes. But your journey will take determination. Each adversary has unique bullet patterns that you must dodge as well (don't worry, it's not that hard). Killing is unnecessary: negotiate out of danger using the unique battle system. Time your attacks for extra damage, then dodge enemy attacks in a style reminiscent of top-down shooters. Soulful, character-rich story with an emphasis on humor. Created mostly by one person. Become friends with all of the bosses! At least 5 dogs.