GOG has made a change to its refund policy, simplifying the process and letting you get your money back even if you've played the game, as long as you request the refund within 30 days.
Previously, GOG's policy was a bit stricter than the likes of Steam's, as refunds were only guaranteed if the game hadn't been downloaded and played. Refunds could be appealed for other reasons, but not liking the game wasn't one of them. Now you don't have to worry about any of that.
If you've played the game, it's fine—you can still get a refund. It doesn't matter how long you've played for, either. Since most games can theoretically be completed in a month, the new policy seems pretty open to abuse, but GOG will be keeping an eye on refunds to make sure it's not being exploited.
"We're monitoring the effects of the current update to make sure no one is using this policy to hurt the developers that put their time and heart into making great games," the FAQ reads. "We may refuse refunds in such individual cases. We'd also let you know about any future adjustments in the voluntary Refund Policy in advance."
The new refund policy is active now.
I previously stated that it was 'no questions asked,' but GOG has clarified that it will treat each refund request individually and ask customers why they are seeking one.