how do I get a refund from gog.com?

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.

https://support.gog.com/hc/en-us/articles/212183849-6-I-want-a-refund-How-do-I-initiate-the-refund-procedure-

Go into your Orders, find the game(s) you want to refund. Next to order number, there's a drop-down arrow, click it and choose "Contact Support". This will take you to a support page that has your order number pre-listed.

Fill in the fields.

The only issue with this form is that refunds are not included in the "Problem type" drop-down menu, so pick the one that seems closest/fitting best. That only applies if the reason for the refund is related to a technical issue. One can ask for a refund, no questions asked, so long as they've not made an attempt to download any of a game's content.