Contact support. Find your game on the left side, explain your issue and GOG will help you get it running. If they can't do so, you are entitled to a refund, provided you send your ticket within 30 days from purchase.
Alternatively, describe your problem in the forum, the community is usually quite good at solving issues.

Go here (or look for the link "Games support" at the footer of each and every page) and look up up your game. There you'll find some support articles, and if none of those work for you, you can contact support through a link near the footer of the game's support page. If support can't help you, you should be able to ask for a refund. Correct me if I'm wrong, but I think the "30 days after purchase" limit is for games you haven't downloaded yet, not for games with technical issues.

From GOG.com Policies, "Money Back Guarantee Policy" section:

Alright, I stand corrected.

Before requesting the refund, may want to mention what issue you;re having with what game. One of us may be able to help you. Stranger things have happened on a support forum. :)

So i got Mob Rules and i am getting an error i have tried all the ways on the help page for this game and it still doesnt work. The error is 0xc0000022 error , the application was unable to start correctly.

i have battletech and its relly unstable i dont want this game anymore i bought it whith the intencion of streaming it and it nmearly cripplis my computer ive tryed all i can to reslave the issue but it makes my computer crash after close

i am an idiot and bought a key i couldn't use.

bought the witcher 3 season pass key via humble bundle and didn't realize it wasn't a steam code. not very savvy with this sort of stuff so i would just like to return it. not sure if this is the right way to ask.
please respond soon.

Hey, I think you'll have to ask Humble about that, since they were the ones selling it.

Humble does not issue refunds ofr games.