I was gifted the Witcher 2 on Steam by a friend, but I'm not sure how to get the key from Steam as it doesn't list it when I look at the Witcher 2 in my games list and I've tried some advice which was looking in the registry for the key, any advice?

Visit http://www.thewitcher.com/backup

In the right collum there is the following text and a link.
I have problems redeeming my product key or my copy of The Witcher 2 didn't come with product key, what should I do?

There you can send a proof of purchase to CD Projekt.

Go to your Library, select Detail View, click on the game. On the right side of the screen, there will be a list called "Links". Look for "CD Key". Click on it and there's your key.

The Witcher 2 doesn't have a cd key on the visible Steam key system (which is used when you need to enter a key into an in-game register window). You do have a key and can registry hunt for it (Steam forums for Witcher 2 will give you directions) but you can also use the link provided above and tell it you don't have proof of purchase to get to the Steam-based resend cd key request (requires your steam profile to be public so they can see what games you have).

Try one of these solutions:
http://www.gog.com/en/forum/the_witcher_2/to_redeem_your_steam_version_with_gogcom

You sir, deserve a medal.