blotunga: ... You can always select USD as your currency. That way the exchange rate's impact is less.
You always have the impact of the exchange rate - I mean even if you buy with USD prices here someone has to convert to your local currency bank account in the middle (paypal, visa, someone else) with the actual exchange rate. Having the two currencies here just means that everyone could always choose the better one. So if GOG by chance undervalues the local currency buy in USD or the other way around.
I don't really expect GOG to closely track the exchange rate but rather apply a moving average over some time window. After all they are not a bank and do not have the time for this. Having local currencies is probably only good if the currencies do not fluctuate a lot. Having worldwide just USD prices was working fine. But you always have the full impact of the exchange rate, which way ever.