Anyone know why the Witcher download is twice as big on Steam? I'm wanting to play through the game again, and am just wondering which would be better to download. Not too sure why it's so much larger on Steam though.

Probably because you download an installer here, while Steam's client installs the game directly.
My Witcher 1 folder uses 14 gigas.

just like what he said, steam uses cloud to stream game while GOG actually lets you download the installer itself.

How do I activate my GOG copy on Steam?

Sorry you can't, it only works the other way around.

You can get a backup copy of both TW1 & 2 here on GOG for no extra cost if you own the Steam version.

http://gog.com/witcherbackup

You can't activate it but you can add it to your Steam library if you want the game to work with Steam.. Open steam then click the games tap near the top right and select "add a non-steam game to my library".

AFAIK, doing that (I tried it once) does nothing except allow you to run the game from the Steam Library interface. No real purpose served by doing that, imo. Or, did I miss something?

You are right. It just lets you talk to people on steam while playing and it keeps track of your time played. Personally I wouldn't do it but if someone really likes Steam and wants to then more power to them.

Because steam sucks donkey's ass. Get your gog copy, here, today!

LOL. I was about to write, almost letter by letter, the same :D

Thanks :)

it says invalid code for me. it is a 8-5-5-5-5 digit code I got from steam.

I think mine did the same thing the when I tried, Witcher 2 worked fine but Witcher gave me the error, but if you look at the right side of the page there is a link:

http://thewitcher.com/backup/

that will give you another way to show you own the game on Steam, and then you will get a key emailed in a few days. That's how it worked for me at least.

Thank you, I will try that way.