How do i go about asking for a refund?

See: https://support.gog.com/hc/en-us/articles/115000487189-Money-Back-Guarantee-Policy

6. I WANT A REFUND! HOW DO I INITIATE THE REFUND PROCEDURE?

You simply need to contact our Support Team, by using the “Technical Issues with Games” form, and describing the problem as accurately as possible. We will get back to you with what we think might help fix your problem. If we can't get it working together, you will get your money back.

May I ask which game and why? Unlike Steam you cannot get a refund if you change your mind... Only if there are issues with the game running on your system that support can't help you fix

It would be hard to get a refund for one of your games you already played even if 30 days period is not expired. Highly unlikely to be precise.

Of course you can get a refund if you change your mind. But only if you have NOT downloaded the game. I don't know if the rules have changed, since the fair prices program ended, but you should get the amount spent, as GOG wallet funds.

Several months ago I got a refund when a trade fell through, and I ended up a code for some gift games.

Ok, but looking at his profile he owns 3 games and has played them all.

You are next level when it come to thoroughness, I bow to your superior skill. ;)

Even on steam he'd be eligible for the first witcher only.

No refunds for not liking a game.