Nuclear84: Well I suppose I can just start using my converter app on my phone for now. But, I honestly believe GOG are alienating a lot of potential customers by simply not having regional equivalent prices on their site, it really is off putting when you're not sure how much a product really is.
Lifthrasil: I'd rather say they attract a lot of customers with their 'one world - one price' policy. So it's good that they use US$ prices for everyone in the world (it is the most common international trade currency after all). And as others have pointed out they can't just select some conversation rate and post prices in other currencies for legal reasons. Because those prices would be binding and GoG would run danger to either make a loss in the conversion (which they can't afford as a company) or of overcharging their customers (which they don't want). So it's better to trust that the customers are able (and willing) to use Google themselves. And most customers are OK with that.
But it's more than just using Google. You have to know to find out what you're bank charges are. So instead of having prices up front for your viewing pleasure, you have to go to google and find out the UK price, then double check the current bank charge then add that to the googled price, then note this down somewhere so you don't forget to soon. Then you have to do this for every price every day that you might want to buy something.
Oh and those bank charges can be more onerous then if GOG just set a (slightly high) transfer rate itself. An extra £1.25 on a £4 game is quite a lot.
To be honest I think this would mostly be a convenience for their customers. Yes it would be more work for them to set up and maintain but the reward is simple and upfront pricing of their games. And to be frank, I trust GOG not to use it as a cover to set up a national pricing scheme.