We say "the GM" all the time like it always means the same thing, game by game or group by group. But it's not.
- rules-applier and interpreter, as in "referee" - in-game-world time manager - changer of scenes - color provider - ensurer of protagonist screen time - regulator of pacing (in real time) - authority over what information can be acted upon by which characters - authority over internal plausibility - "where the buck stops" in terms of establishing the Explorative content - social manager of who gets to speak when
A given role-playing experience must have these things - therefore is no such thing as "GM-less" play. But which of these require(s) enforcing varies greatly, as does whether they are concentrated into a particular person, and as does whether that person is openly acknowledged as such. GMing Task-Distributed play is certainly possible.
Not all instances of role-playing include the same GMing tasks or organize them in the same way, so what the GM does in game X may not be the same thing in game Y. That's why assuming special properties into the term "The GM" causes a lot of problems, especially when it's supposed to include all the tasks.
Narrativism: Story Now includes a list of GMing tasks.