Mooszek: So retail copy won't be connected in any way with copy on GoG? It is only the same cd key, but two different copies and I can play retail copy even if I lose my GoG account, or I can play my copy from GoG even if I lose my retail copy?
The post I linked is for the following case.
You have The Witcher 2 on GOG.com, with cd-key AAAA-AAAA-AAAA-AAAA and a retail copy of The Witcher 2 with cd-key BBBB-BBBB-BBBB-BBBB
Trying to activate the retail copy on your account will obviously fail, since you already have The Witcher 2 on GOG.com. Thus you are able to allow a friend to enter the BBBB-BBBB-BBBB-BBBB key on GOG.com and get The Witcher 2 on GOG.com with the cd-key BBBB-BBBB-BBBB-BBBB.
I think that the cd-key is only used to make an account on
, but I may be mistaken. In case this is correct, if you have activated your retail copy on [url=http://www.thewitcher.com]thewitcher.com, your friend won't be able to activate there.
Disclaimer: Still haven't installed The Witcher 2, and I only have a GOG.com copy, so any part of the above may be wrong.