Little late here but whatever. The GM idea was originally suggested by Bubbet after Pronto brought the issue up on the the discord. I then expanded on it so we have the version that Pronto posted in the original post. Only difference is I described them as an Arbiter for 2 reasons. Reason 1 I'd been playing through Halo 2, reason 2 it sounds cool.
The general idea was based around my experience being the GM for my D&D group. In D&D you have the established rule book but it functions as more as a suggestion for the GM. In the end if the book says one thing and the GM says another you go with the GM. Not only is it their job to be the rules, but the world as well. The GM spins the world and the story's with in. They are not just the god of rules, but the story teller adapting to whatever their players do.
That is how we landed on the idea of a GM. We realize that many different people participate in RP in many different ways. There was no way we could create a set of rules to encompass everyone. Instead we have the GMs to make sure everything is fair.
I'll be in here trying to answer some questions that have been asked already and expand with some possible alternative
ideas to Pronto's answers.