Leroux: Did you buy the game here on GOG? If not, no fair price package. If yes, check the store page of The Witcher 3 here on GOG. If there's something like this written under the price:

$6.80 OFF ANY FUTURE PURCHASE. You get bonus store credit (our Fair Price Package) to offset the regional pricing. (you might see a different figure)

Then you'll get that sum subtracted from the asking price if you ever decide to buy a game on GOG again. E.g. a game usually priced $9.99 here, would only cost you $3.19 in the above case. So no money back, just a store credit that makes the next game you buy here cheaper for you.

You might be able to check it out by putting a game in the shopping cart and going to checkout just to see if something is subtracted from the price, then cancel that purchase instead of confirming.

Ferryman95: As an update. I was entitled to a couple of Euro buying the game. But i'm not sure if it was the same when you bought it pre-order. That and the fact that i had (USD) as chosen local currency in GoG Galaxy I've decided that it's most likely my fault. Just the fact that they have this service is an amazing thing, and they can keep the money.