Try to buy it if you can , it gives you absolutely no choice between GOG and Steam - since there isn't one . Apparently the Steam Icon is a "visual glitch" and there are only keys for GOG . Me and some other guy have the same problem and it's on Humble Bundle , not us .
Edit : 100% Bought the Witcher 3 GOTY Edition , not the normal one
In that case, you should ask for a refund from Humble as the text on the storepage is false advertising.
The Witcher® 3: Wild Hunt GAME OF THE YEAR EDITION is provided via GOG for Windows and Steam for Windows. A free GOG or Steam account is required.
Since the Steam version is only officially available through Steam, your only hope is if someone got Witcher 3 as a Steam gift. Otherwise, you're likely to wait till the game is discounted again on Steam (likely summer).