I took three healers starting at about ~4k GS through gammas, with tanks of various levels, including sub-4K. The only wipes were from garbage DPS who didn’t kill Mirror Images, because Gammas are tuned for Naxx gear. He will 100% be fine to hold aggro, it’s about being aggressive and pushing in the dungeon and not stopping for a lunch break every two pulls.
I so rarely had to hardcast on the Priest or Druid, that it actually caught me off guard when I did.