Does the function from "HttpService" GenerateGuiID always return a identifier that has never been used before?
Example: If someone in another game called GenerateGuiID would it never return a identifier that was returned by GenerateGuiID in another game.
Not necessarily, but you're very unlikely to ever run into that problem.
To quote wikipedia:
"Assuming uniform probability for simplicity, the probability of one duplicate would be about 50% if every person on earth as of 2014 owned 600 million GUIDs."
I don't know how it is implemented for ROBLOX, but you're very unlikely to ever run into duplicates in a sane amount of time.