GOG has updated its voluntary refund policy effective immediately giving people the ability to refund any games purchased up to 30 days after the purchase date, and it does not matter if that game has been played. Obviously, such a system could be open to abuse due to the generous amount of time given so GOG has stated it will be monitoring behaviors of users and will reserve the right to refuse refunds if there are suspicions of the system being abused. In the FAQ this is what GOG says:
We trust that you’re making informed purchasing decisions and will use this updated voluntary Refund Policy only if something doesn’t work as you expected.
This is why there are no limits but instead, we reserve the right to refuse refunds in individual cases.
Please respect all the time and hard work put into making the games you play and remember that refunds are not reviews. If you finished the game and didn’t like it, please consider sharing your opinion instead. Also, please don’t take advantage of our trust by asking for an unreasonable amount of games to be refunded. Don’t be that person. No one likes that person.
Outside of possible abuse, there are no caveats. People just need to simply contact the customer service team to request a refund, but GOG has stated that if the reason is due to the game not working then they can first work with you to fix issues. The refunds themselves can be issued back in two ways. The first is a refund to as credit in the GOG wallet which would be applied instantly. The second option is a refund to the method of payment so the money is sent back to your account, and that could take a couple of days. However, payments through paysafecard, SOFORT, and GiroPay can take a few weeks to be refunded. If you are gifted a game you cannot apply for a refund, only the gifter can get a refund. DLC can be refunded too through the same system.
So, TL;DR: 30 days no quibble refund, don’t abuse the system as no one appreciates that, DLC can be refunded, and you can’t get a refund on a received gift.