Thanks for the giveaway;)

It's another game with messed up Steam API.

Both Steam and SG show, that I own it, yet I can enter the GA... and it seems, that rightly so this time, because what I own, according to history of purchases on Steam support, is the previously free, but removed from the store, Offworld Trading Company - Free Multiplayer without Offworld Trading Company - Core Game Upgrade.
Installing right now to make sure, what's up., because it's so messed up though, that even SteamDB reports it as the full game.

Yup, confirmed, it's the F2P multiplayer version. Singleplayer "Requires purchase of the whole game"

ty