Yea, her and her unit become immune to anything. While nice, GK have plenty of other psychic powers which boost their stats, so its not like it would really be game changing. Sure every army has the ability to take some kind of psyker, but whether or not you'll face one is questionable, not to mention there will be plenty of other units for the psyker to target besides her. I think it really just saves her (and her squad to some degree) from force weapons and the occasional sniper psychic ability.
As for malediction, I've had mixed results. In one game it killed 2 justicars and a librarian (very rare, but DEFINITELY paid for itself) and in other games I've had it completely wiff. I think its only worth taking if you have your build down cold, and have 20 extra points to spare. I would never build a force around it. Even then you'd probably do more damage with a shattershard for 5 points less.