I think you'll need a Steam key from Gamersgate to download anything through Steam.
Steam keys can be sent by email, or sometimes by logging into the site where you bought the game from and looking at your purchases.
Check here where someone asked the questionhttps://steamcommunity.com/app/267900/discussions/0/648817377954312724/
Log into GamersGate, click on "My Games", beside the title should say "unused serial) > click on the game and you should see a button below the download one, click it.
The OP said
I guess I didn't see it before because it didn't come in until now
so maybe there is a delay between when you purchase the game and when the Steam key appears at Gamersgate.