Gergiev's Mahler isn't 'bad' by any means, but one would do better picking up the Bertini, Bernstein (any of them) or Zinman box sets of the complete Mahler symphonies instead. Even the Rattle Mahler box - weirdly conducted as it sometimes is - gives one more to think about (however, EMI left out Rattle's Berlin remake of M9). Also, a tremendous amount of 'bang for the buck' can be had with EMI's "Mahler - Complete Works" box set. That's the best deal of them all.
I know there was plenty of pooh-poohing of Zinman's complete Mahler, but I think it's miles ahead of Gergiev's - being both better recorded and interpretively more 'central'.