OK, it's been a few days, and nothing else has come up. I'd really like to play this game that I can tell would be tons of fun if it would work, but I don't think I can if it's not functioning. What's the procedure for GOG refunding a game that doesn't work?
You need to speak to GOG support staff for help with your issue. It wouldn't hurt to speak with the Unepic developer via the official site, too, as Unepic is a game that is still getting new versions so if there is a bug then they might be able to fix it for a future release.
As for refund, I very much doubt you'll get that.
Firstly, the game you download from GOG is DRM free, so you can use it on any number of machines at any time, no activation needed. So once you've downloaded it (as GOG can tell once a game has been downloaded) it is in your possession to do with as you will. If the game had not been downloaded then GOG know you don't have it and can refund it, but this is usually only needed where errors have occurred in the purchasing transaction, such as buying a game instead of buying a gift or cancelling a pre-order. But once you've downloaded a game, you have it.
Secondly, how can you prove to GOG that you have the issue? How can you prove it is the game and not your machine? As said above, you have the game installer and can make as many copies of it as you like, so for you to get a refund you would have to prove that you have deleted all copies that you have. This cannot be done. So there is no refund policy here as from GOG's point of view you could just be looking to get your money back whilst keeping a copy of the game.
Here is the refund policy: Refund Policy
You can cancel a preorder and receive a full refund if you request this before the preordered game's release. The refund request becomes invalid if you attempted to download the game, or its bonus content.
You can request a replacement game of equal or lower value (for games purchased during discounts, the discounted price applies) if the request is made within 7 days after purchase, and you have not attempted to download the game or its bonus content.
A sale is considered final after 7 days after purchase, or when you make a download attempt for a game or its bonus content.