Bought The Witcher 2 from Steam for $3.99.
You can use the first method to redeem a GOG copy, but you have to install all 15GB of it first.
On first run, Steam will allocate a key for you and then you have an option to copy the cd-key to clipboard.
I copied the key, then went to GOG and redeemed a DRM-free copy.
Hope this helps though those with bandwidth limits will have to be careful since you'll be downloading 30GB(+10GB for bonus materials) this month.
Strange, i didn´t install The Witcher 2 and my cd key is visible.