Hello there, I was playing the game via Steam, but now, im unable to login because I also have one more account via GOG using same email address with Steam. How can i revoke my steam's gwent account ?
I did also try to change username of my available account to my old account"s username, it says username already taken

Of course you lost, you were playing The Game!

What did steam say about you not being able to log in on their site?

There's no such thing as a "Gwent account". There are only GOG accounts, which are required to play Gwent: The Witcher Card game, regardless of the platform (GOG, Steam, Android, or iOS).

If you want to delete one of your GOG accounts, see How can I delete my account?

Edit: you might also want to read Gwent: The Witcher Card Game FAQ.
If you're having a Steam-specific problem, post either on the Steam discussions page for the game, or on the CD Projekt Red forums.