GOG/CD Projekt are in Poland which does not use the Euro BTW, they have their own currency
. Aside from that though they most likely do not convert every form of received money into Zlotys nor Euro, but most likely either into USD or keep it as they receive it to some degree or another.
This is all true but only for their physical location.
From their new User Agreement:
"1. ABOUT THIS AGREEMENT
1.1 This Agreement is a contract between you and GOG Ltd, 7 Florinis Street, Greg Tower, 6th floor, 1065 Nicosia, Cyprus
and applies to www.GOG.com, your GOG user account, the GOG Downloader, the GOG Galaxy Client for updating games, any games or videos or other content which you purchase or access via GOG.com, the GOG web forums, GOG customer and technical support and other services we provide to you (we'll just call all this “GOG services” for short).[...]"
I wonder if Treasure ever went to visit that building to see what's there...