I bought a key for Mount & Blade warband. if anyone has a key for steam ill trade that for this one for GOG

If you haven't downloaded it yet contact support.
They'll refund you.

Bought it on G2A was unaware GOG was a website and found out after I payed, I contacted support but hy hopes are not high

A GOG key from G2A is pretty much worthless.

GOG generally doesn't allow resellers, and has never allowed for G2A to be a reseller. There are lots of claims of G2A bought games being removed from people's libraries.

Ah, just as a warning it's very unlikely you will be able to get rid of that key as GOG doesn't let others resell keys aside from The Witcher GOG keys on humble.
You can try to trade for it here though:
https://www.gog.com/forum/general/gogs_general_trading_thread

Thanks