OldFatGuy: I don't mean to be an ass, but if it used to take 5.69 Euros to equal 5.99 dollars but now it only takes 5.49 Euros to equal 5.99 dollars, then the value of the EUR INCREASED, not decreased. It takes fewer Euros to purchase the same amount of dollars.
As I mentioned before, the whole issue is much more complicated than this. Telling the community that they've changed their prices because of conversion rates is no lie but only half the truth.
For example, GOG is booking its Euro income via a sub company in Cyprus, probably to save tax. GOG itself is localizied in Poland, so to pay their local bills, salaries etc. they've got to exchange these Euros to Zlotys. Over the last years the conversion rates between Euro and Zloty have been rather stable, hence was their income form Euro countries. The USD on the other hand has gained in value compared to the Zloty. Thus actually GOG in Poland earns more money now from USD income, because they get more Zlotys for each USD atm. But instead of lowering the USD prices, they raise the Euro prices to earn more money from there as well. As sneaky as it seems at first glance, there's also fair and square reason to this. Not all of GOG's expenses are paid in Zlotys or Euro. Especially game distributors outside the Euro zone are likely to charge their share in USD. While this is no problem when customers purchase the games in USD, GOG effectively has to pay a bigger share when customers buy in Euro. From this point of view it makes perfectly sense to increase the Euro prices. Hence GOG's income is dependent on the ratio from where the distributors come from as well as the ratio which games are bought in which currency.
There are even more factors involved, but all in all I suspect a hidden rise in prices, maybe because of risen costs, e.g. due to the development of Galaxy and whatnot. GOG is a public company so anyone who wants to cross-check this theory might find some answers in the companies public annual reports.