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 $196.4K — a different platform, not this one.

METAL SLUG 2

“METAL SLUG 2” , the 2nd entry in SNK’s emblematic 2D run & gun action shooting game returns to the battlefield on the GOG gaming platform! ARCADE MODE (MAIN MISSION) - Play the original arcade version of “METAL SLUG 2” from Missions 1 to 6, and fight your way through the hail of bullets to thwart General Morden's evil ambitions, and uncover the secret of his new coup!! MISSION MODE (STAGE SELECT) - Play, and replay your favorite missions as much as you want! Hone your skills alone or along with a brother-in-arms on the levels you have unlocked so far! SETTINGS - Set your controls, language, video display, and sound volume settings for the ultimate “METAL SLUG 2” experience on PC! LEADERBOARDS - Check your current rank & score in “METAL SLUG” at any time, and challenge other players to be the best “METAL SLUG 2” soldier in the world!