Can I submit GoG codes?

https://www.steamgifts.com/about/guidelines
"Giveaways
Creating
All giveaways must be for Steam redeemable gifts or Steam redeemable keys."

You can let some random, ungrateful ninja grab it in the orphan keys topic if you want. Just say it's a GoG key.

You can't make a Giveaway with it but you are able to drop it in the discussions.

When you encrypt the key with as example rot-13 are the chance good that it reach a real user.
You can drop it at each discussion/thread and i would NOT recommend the orphan thread because it is overwhelmed with key sniper bots.
You are able to make one by yourself or as example post it in this thread. All up to you.

Ps.: Your brezel face avatar looks strange :-D

Let me guess: it is the Witcher code, which is currently given away for FREE?

if you choose to give it with drop on this site I suggests you to use more hard coding way than rot13 you have tool very usefull :

https://paulschou.com/tools/xlate/
you can combine with :
https://www.dcode.fr/code-morse or https://www.dcode.fr/liste-outils

I find these tool here :
https://www.steamgifts.com/discussion/TLR8m/zelgh-magic-guide-to-basic-sg-puzzling-update-october-2017

or other method you can create topic open for X time and say something like all want chance to win game X say enter and at the end you use tool for generate a random number for choose winner and email the key!
have a good day

The GOG Community Gifting Thread

Man, it is STEAMgifts.com.