GR00T: The game prices are based on US dollars. If a game costs 5 bucks US, GOG uses the current exchange rate to post the price in CDN. So, whatever 5 USD is in CDN bucks is what we pay: the US dollar equivalent in CDN dollars.
77 cents is roughly 30% exchange rate, so we'd pay 5 bucks + 30% of 5 bucks in CDN, which equals 6.50 CDN.
It's more complicated than that actually. The USD price that someone is shown depends on what country they are in and also varies game by game. So if you connect to GOG from a USA IP address and have USD as your currency you will see one price. If you connect to GOG from a Canadian IP address and have USD selected you'll find that some games have the same USD price as they do in the USA, and other games have a higher or lower USD price depending on how the publisher of the game decided to price the game.
Additionally, some games have GOG's fair price package which gives you a discount applied to your GOG wallet I believe with the purchase of some games.
Selecting CAD instead of USD does not result in a unified exchange rate between USD/CAD to every game either. Additionally when they do apply whatever exchange rate they're using they then round it upward to the nearest $0.09 also so that the prices always end with a 9 instead of a 0. This is also why some games say up to 85% discount but when you put it in your cart often it is only 84%, because it is actually something like 84.98% due to the 9 rounding happening and they truncate the decimal (or so it seems by my calculations) leaving it at 84% or similar.
All of this stuff can vary from one game to the next with no universal formula. If anyone wants to see the prices as they are in another country you have to change the gog_lc cookie in your web browser. Part of the cookie data is the country you are in, the other is the currency and finally language.
CA_USD_en -> Canada + USD currency
CA_CAD_en -> Canada + CAD currency
US_USD_en -> USA + USD currency
If you compare CA_USD with US_USD or other countries xx_USD, you'll find that the USD prices themselves vary from country to country as I mentioned above, but only on certain games, and the difference might just be the way it is, or there might be a fair price GOG wallet cashback attached (see store game card for game). Furthermore, fair price packages might exist in some country(ies) and not in others IIRC for a given game.
They do not track the exchange rate on a daily basis either, but rather they update the exchange rate they're using for a given currency conversion whenever it changes on the open market by a threshold amount. This is to avoid the prices of games sporadically changing by the minute/hour/day too often. Normally they wont change more than once a month or more due to the granularity they use.