Game instance is very like Game Mode, Player Controller etc etc.
It’s one of these blueprints that’s constantly available in the background.
But with the GI, you get one when the game starts running, and that one exists for the duration of the game. Exactly what you need if you ask me.
Added bonus, sort of, if you talk to it with blueprint interfaces ( rather than directly access the innards ) you can use it at any time without having to cast… 