The guardian could be Alice. The switch in scenes from Belle mentioning the guardian to Alice jumping out of the portal and almost knocking Rumple over could be a hint. It took a while to find each other because Rumbelle were on the edge of Realms (a Realm on the edge of Realms???) along with this new Cinderella, Henry etc, and Alice had to have a reason and the ability to get there. I don’t think she is Rumple’s guardian, but perhaps the guardian of the universes/realms/the natural order of things, a job that might pass from one person to the next, like the Author.