G2A sells legitimate keys in trade with vendors, then they have brokered keys by third-parties. G2A themselves has an paid by the buyer policy, G2A Shield, to provide additional protection against ‘problematic’ brokered keys, it is the very same principle when you are trying to sell your own digital goods/accounts. I'm quite sure the G2A deal in mention is in partnership with GOG.
No, a gog rep said that there are no authorized gog key resellers (link).
I know the keys sold by G2a are legitimate. If you live in Bulgaria which falls under EU pricing but has like 300 Euro monthly average income, then by all means buy the key on g2a/kinguin/gmg. Same goes for South Africa, the video game prices in SA are very high compared to the average income. Those keys are intended for you, no harm done. I however cannot justify buying those keys if I live in a country whose average wage is 5-10 times higher than the countries those keys were intended for.
Anyway sorry for derail, let's get back to discussing witcher 3.
Get someone with a CD drive to build an .ISO from your disc for you? copy the .ISO, mount it on your windows and off you go?