Here are the six possibilities that I know of to redeem your Steam copy with GoG (the first 4 solutions may not work for you):

A) [this functionality may not have been provided by Steam for this game]
1. Go to your Steam Games Library
2. Click on The Witcher 2
3. Click on CD Key under LINKS to the right
4. Copy your cd-key from the popup window
5. Enter your cd-key here: http://www.gog.com/en/witcher2/backup/

B)
1. Press the Start button, type 'regedit' and press enter
2. Navigate to HKEY_LOCAL_MACHINE --> SOFTWARE --> Wow6432Node --> CD Projekt RED --> The Witcher 2
3. Check if cd-key is listed on the right
4. Enter your cd-key here: http://www.gog.com/en/witcher2/backup/

C)
1. Press the Start button, type 'regedit' and press enter
2. Navigate to HKEY_LOCAL_MACHINE --> SOFTWARE --> CD Projekt RED --> The Witcher 2
3. Check if cd-key is listed on the right
4. Enter your cd-key here: http://www.gog.com/en/witcher2/backup/

D)
1. Press the Start button, type 'regedit' and press enter
2. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\CD Projekt Red\The Witcher 2
3. Check if cd-key is listed on the right
4. Enter your cd-key here: http://www.gog.com/en/witcher2/backup/

E) [could take up to 72 hours, receipt must be printed]
1. Go to http://www.thewitcher.com/backup
2. Click Digital Version
3. Click Yes when it asks you if you have proof of purchase
4. Print out your Steam purchase email and follow the rest of the instructions.

F) [could take up to 72 hours]
1. Go to http://www.thewitcher.com/backup
2. Click Digital Version
3. Click No when it asks you if you have proof of purchase
4. Provide your public Steam ID and Steam email address so they can check your account.

A didn't work for me, B did. Thanks!

Glad it helped!

I've tried all of the methods and I'm having to rely on method F, I did that last night and I'm hoping the key will arrive soon, it said within 74 hours but I'm hoping it comes sooner.

I too have to rely on F. There's no CD key in the registry and W2 happens to be one of perhaps 4 games where I didn't get a receipt mailed to my due to Steam's mailer being down at the time of purchase.

I'm wondering how long method F would take? I know it said that it would take within the next 72 hours, but I'm wondering if anyone who has done method F has already got their key?

I went with:

B)
1. Press the Start button, type 'regedit' and press enter
2. Navigate to HKEY_LOCAL_MACHINE --> SOFTWARE --> Wow6432Node --> CD Projekt RED --> The Witcher 2
3. Check if cd-key is listed on the right
4. Enter your cd-key here: http://www.gog.com/en/witcher2/backup

But it was listed as Serial not as CD-Key. Keep that in mind that it might actually be worded differently.

I think they mean the key will be sent to the email address, I'm hoping anyway since my emails are different for Steam and GoG.

This is why i am concerned :
Email address provided by you is linked to the back-up copy that you will receive from us.

Does this mean that they will in their database register this email and Steam account ID and send us a key or does this mean the will give us account with game and entered email address?

@dex3108

How I interpret this is that, CD Projekt RED will check that you have the game and contact Valve to make sure the email provided is the correct one for the account, then, they'll email you a serial key to enter at The Witcher 2 backup site so you'll be able to add it to your GoG, regardless of if you use a different email for your GoG account.

i just hope they answer something about it, i mean i got the mail wit that 72 hours thing but if this free backup is just for today i would like to know what happens if they don't send anything else today and tomorrow i get email saying my copy is verified or something like that.

steam is great for its cloud saving but with games like witcher ill rather have them on gog so i can play them in 5 or 10 years without thinking about is the service still alive or when i'm somewhere without internet access (yeah, sometimes i'm without internet access and these gog games are great wit the no drm policy)

Was also forced to use F oh well ^^. Was nowhere in the registry nor did I have a cd key option in steam, wierd.

All I got so far from option F is a confirmation email. I'll let you guys know if I make any progress with this route.

about 17 hours since f email confirmation and not a sing of the key