You don't via the Giveaways page. It requires the gifter to enter a link to a Steam Store page, so users should have the reasonable assumption that all giveaways are for Steam keys.
I recommend joining a Discord group such as GrabFreeGames, as they have a channel called "keydrops" where users can post whatever kind of key they have. Otherwise, you could post to the thread here:
Keydrops Go Here! , clearly stating that it's a gog code. Either way, you'll want to disguise your posted code in some way so that web bots don't snag it, such as "ABCD-EFGH-I??L, ?? = Just Kidding".
Thanks for asking and not just posting it to the Giveaways page for Steam keys since it's annoying to have to check every giveaway on whether it's for Steam or not, when they all should be.