Since you don't get a Witcher cd-key from GoG, one can't register the game using Register.exe.,thus forum access is limited.

Is there any way to remedy this problem?

I'm especially interested in using the Save Game Market
http://www.thewitcher.com/forum/index.php?topic=23708.0

You don't need to register to play. I'm fairly certain the register option on the launch window is a holdover from the retail DRM version. Just launch the game and play.

But you do need to register the game to access certain areas of the official game forums. For games with multi-player that need a key, you need to request one through emailing gog support. try doing that.

Just received this answer from gog support after requesting a key:

[i]Hello,

Publisher of The Witcher (CD Projekt) decided that, since our games are DRM free, there is no need to include a CD-key in our version of the game and did not provide us with any keys. Registration does not bring any benefits so you are not losing anything. Happy gaming :)

Regards

Ethaar
GOG.com Support[/i]