Don't even try. I bought it in a sale for pennies, so no point in requesting a refund. Hopefully GOG will fix it one day. Who knows?
This is a brilliant and worthy successor to the first two games, even though some elements are missed (e.g. Holy Water and Rope arrows, various potions, swimming). However, it can be a bit buggy - some of the locks won't work. So search for the free Sneaky Upgrade patch which smooths this out and adds in cosmetic elements to give it more of a feeling of the first two. That's not supported by GOG but it works fine.
