Can you still do this if you link accounts?

No.

As you can see by clicking GOG Connect near the bottom of the page, there are no eligible games (and there haven't been in ages).

Staff response

If you bought The Witcher or The Witcher 2 on Steam before March 16th, you can get a free GOG copy. Apparently there's no such time limit if the games were bought for PC or Mac at other stores.
Even if you don't own The Witcher at all, you can simply add Gwent: The Witcher Card Game to your account, then claim the Gwent welcome bonus which includes the game.