Hey i earned a copy of Witcher 3:The Wild Hunt for GOG but i prefer using Steam(all my games are registered in my Steam account,it would make things easier).I've never used GOG before and I am willing to trade my account which includes the witcher 3 for a steam CD key of the same game.If anyone's interested,please contact me.

There's not such a "steam CD key". What you might find, is a person willing to gift you a Steam copy of the game, as a Steam gift.

The Steam version of the game can be purchased only via Steam.

Not only are you trading the GOG version, which is considered worth quite a bit less since Nvidia vouchers are available in the trading scene for about half the price of the game, but yours is devalued even more because it's already linked to an account.

On top of that, there are no Steam keys for Witcher 3. The only way to get it, is to buy it on Steam, which means the price value of the Steam version is pretty solid and close to full price.

I would be extremely weary of getting scammed if someone claims they're fine making this trade, because it doesn't make sense. If someone has the funds to buy the Steam copy, they may as well have bought the GOG copy.

click on "Games" in the top left corner of Steam then click on "Add a Non-Steam game to my Library"
by using this feature you can add Witcher 3 to your Steam library & thus solve your problem.