An idea about how the Gravemind learned of the Ark's location that sort of makes sense is that it sifted through Regret's memories and learned of the Luminary featured in one of the Halo 2 Anniversary's prologue Terminals - the one which told Regret about Installation 05, and the portal to the Ark located on Earth. It requires a bit of hoop-jumping, but it sure as hell makes more sense than just having no explanation at all that Halo 3 gives us. As for how Gravy got to the Ark, it was confirmed in Canon Fodder that it used Precursor neural physics travel to jump High Charity to the Ark while in Mars' airspace.