Uhm, where exactly do I find my extra copy of the Witcher 2?

I bought the game on GoG.com and the site

http://www.gog.com/en/witcher2/backup/

isn't helpful at all.

"The Witcher 2 Enhanced Edition is already on your GOG.com account! You're all set and you can download your DRM-free backup whenever you'd like."

I thought you would get a key which you can send to a friend, so that he can unlock the game on GoG.

no its for a backup copy if you bought somewhere other then GOG

As noted, it's for your own personal backup purposes only. You're not supposed to give it to someone else.