Are you talking about the fact that store credit only lasts for a year?

Are you kidding?

No, when you refund something they always make it very clear that store credit is only good for one year. I'm not sure what the reason for that is, but that's the way it works. That said, in most (if not all) cases you also have the option of a cash refund, so if you don't plan on buying anything within a year you can just go with that. This is the message they usually send in response to a refund request:

"Please let me know if you wish to receive a cash refund, or if a conversion to store credit (which you can use to make a purchase on GOG.com) will be sufficient.
Cash refunds take a couple of business days to be fully processed on both sides, whereas store credit gets assigned immediately. Please note, however, that store credit is only valid for a year."

No i am not kidding. Can you think of one reason why the store credits have to expire after one year? A reason that is related to technology, security or that is beneficial to the customer? Sure, if it was a physical store i can understand it, with store credit receipts floating around, it's impractical. But for an online store? It's like saying "hey, because of reasons, your account will expire after one year!". The database is still there, why can't my credits still be there as well?

Very strange,it must be costing them money to keep the account (credit) updated.

Sure. Carrying a ? on a balance sheet isn't any fun come tax time. If management decided they aren't going to put up with that hassle, then they expire credits so that credits can exist in the first place.

Not every good reason feels good. You get store credit. Wasn't always like that, and isn't like that everywhere.

Its purely an accounting thing. They want it off the books and the added fact that some people will just let it expire instead of using it. Especially odd since that costs them nothing but whatever bandwidth it would be to download the game. Generally speaking I doubt its much more than book keeping, but since its all digital and costs them near zero, its somewhat shady to expire them in my book. In fact local consumer laws around where I live wont allow that at all for retail stores. Gift cards or credit or whatnot are good for the life of the company.

It probably boils down to them not wanting to have some x amount of indefinite store credit that for one reason or another never gets claimed. This way, it expedites the use and it clears the accounting books within a timely fashion.

Yes. If credit doesn't expire, the store offering it has to be treated as a different entity, more specifically a bank. That opens a whole different can of worms, both for the store and the customer.
Luckily, GOG keeps your store credit valid for a year since it was last updated. Buy something that gives you store credit and your current balance will still be available, if not increased. Read 6.11 in the user agreement for more.

Complaint posted.

Also they have to pay taxes on the credit.

While I do not know anything regarding the accounting principles or taxation regulations of Warsaw, in the United States (last time I checked), taxes on store credit (at least for retailers)--while they do have to pay taxes--recognition of this revenue can be deferred up to two years in some cases. You would still need to carry the "?" or liability on your balance sheet--unless your year of credit starts exactly when Gog's fiscal year begins.

I think that Gog is just being flexible to allow credit when in some cases a refund cannot be issued. They also want to motivate those with store credit to use it, so a year's time was arbitrarily established.

That said, if they offered a choice, your best option would be a refund. I believe the reason they offer that choice is they are a for-profit company and would just love it if you were nice and saved them the cost of the refund transaction and guarantee their revenue (at a later date). I personally had this happen, and was in such a giving mood gambled that I'd use the credit in a year's time. Because... Gog!

Funny that OP marked nightcraw1er's post as the solution, rather than any of the various posts that actually give explanations why this happens. :/