Poking in here too, though I'll just say the same things I said before, and which have largely already been said.
1. Imagine you went to your local supermarket and they asked for ID at the register and determined prices depending on where you lived. But they don't do that, you could even come from another country and they wouldn't care, they charge you what they charge everyone, because it's the exact same product sold by the exact same store. But apparently, if we're talking of a stream of bits sent over a network, the direction they travel in changes the price.
2. The VAT is just an excuse. GOG has their financial services located in Cyprus, as you know, and they pay Cyprus VAT of 19% as a result. As of next year they will pay different VAT depending on the buyer's country if higher (which is an absolutely illogical law, mind you), but no matter what they are paying the 19%, so at the moment it's just an excuse and as of next year it'd only justify an difference of at most a few percent, definitely not 1 USD = 1 EUR or such. Also, EU law states clearly that prices must include VAT, and GOG is an EU company, so prices must include VAT, can't be listed separately.
3. I'll just put
this here again. So basically there's little sense or reason to any of it and devs just go along with what Steam says, which you just proved now as well. And you may notice that the VAT barely got a line in those 3 pages, sort of "well, might as well mention this in passing too I guess", but it doesn't justify much of anything.
4. As others have said, "one world, one price" (and the US price at that, not hiking to the EU price as Disney did recently for some newly added games) was a core value of GOG, one of the two clear, specific ones listed since they launched back in 2008. This year they decided to give up on it. That won't fly with the, if I may, core audience that came here for the things that made GOG, well, GOG, different, more than just another shop.