Tough to decide. I guess I'd go with a strong CR 3.
So would I.
Incidentally, if they are immune to gravity that means they should be unable to Dive (as they don't have weight to accelerate them, which infers they can't make Dive attacks either) and Climb at their full speed (as they aren't "moving up").
Should we include that somewhere, in tactics or the Immunity To Gravity. I suppose we could make it a separate "Weightless Flight" SQ, but that seems superfluous.
For example:
Ni'iaths start combat by trying to catapult an opponent into a nearby object (boulder, tree, et cetera) with their tail-sting attack.
A ni'ath's immunity to gravity means it cannot make a dive attack. Ni'iaths never catapult prey anywhere they might lose it, such as over a cliff. The ni'iath pack then closes in on their chosen prey, either finishing it off with their claws and teeth, or carrying their victim off to their lair.
Immunity to Gravity Effects (Ex): A ni'iath is immune to any gravity-based effect (such as a reverse gravity spell or falling damage). It can perform all actions (including flying with perfect maneuverability) without penalty whether it is in light, heavy or normal gravity.
Flying ni'iaths are unable to dive or make dive attacks, since they cannot use the pull of gravity to speed their descent. An inactive ni'iath simply hovers in the air, drifting with the wind until it rots away or is eaten.