It COULD be Gallifrey, but a proto-Gallifrey. River Song obviously knows how the TARDIS works, so she could make it drag a planet without six pilots. Plus, Amy obviously knows something, even a little, because she was supposed to land the TARDIS. Every Dr. Who enemy could be converging on it because when Gallifrey emerges from the Pandorica the Time Lords as we know them may not exist yet, but they can see through the TARDIS cloaking device due to some wibbly wobbly timey wimey stuff and be inspired to create them, essentially making The Doctor both the creator and destroyer (twice) of Gallifrey. The Gallifrey that the Doctor creates creates the Doctor, and that's why the Mighty Morphin' Power Daleks and every major Dr. Who villain are trying to stop it.
Or it could be a Piece of Eden.

Just for laffs.