I have installed my copy of the witcher and when i try to enter the gog.com code it says "code already used"

I bought the game from game stop.

Then you need to contact the support.

For this kind of problems you should contact GOG support or Gamestop support. But, if I remember well, there has been a member of the forum with a similar issue. Perhaps you should look for his thread.

Or maby even the CD Projekt RED support since it was their key.

You bought the game from Gamestop, so you need to contact them. If you get no satisfaction, demand your money back.

Thanks for the comments. I guess I'll start at the local game stop and see what they have to say.

Here is the thread where someone had the same issue:

http://www.gog.com/forum/the_witcher_3_wild_hunt/game_key_says_its_already_been_used