Do these exist, and where can we buy?
Which version are you looking for? On the site it shows ISO versions available as well as some that have the ISO enter key included. Let me know which version you are looking for and I will see if I can give you a link
Any of the sets, but likely would get the ash or white.
From what I’m seeing, the GPBT keycaps are unfortunately only for ANSI types. The Aura V2s and regular caps have ISO support. Unfortunately if you are looking for the gradient styles it appears you are out of luck.