Can I make a giveaway for GOG key?

You can't make giveaways, if the games from GOG platform. You can just giveaways games from steam.

Only games that can be activated on Steam can be given away here. This site works by comparing the games you won here to what you own on your Steam account that is linked here. The site will prevent you from entering giveaways for games you already own and you must activate any game you win here on your Steam account. If someone wins a game here and marks it as received without activating it on their Steam account, that is against the rules and they will end up receiving a suspension for it. This prevents people from entering giveaways to get free games for the purpose of regifting, trading, or selling. Everything calculated here is based on our Steam accounts and the cost of games on Steam.

You can always give the key away in the discussions to someone who may want it, but you will not get contributor value added to your account here unless you create a giveaway and provide the winner with a game that can be activated on Steam.

Welcome to steamgifts :)
If you want to learn a lot more about how the site works, pretty much everything is explained in the FAQ and guidelines.

Thank you for fast and good answers!

On goggifts.com

A couple of useful links (you can also find them in the Help menu at the top):
https://www.steamgifts.com/about/faq
https://www.steamgifts.com/about/guidelines