As someone who uses rebels to easily kill weaker gas teams (and still defeat the very best, but drop one to rng once in a long while when they are geared, modded, and reliced to the nines), without that zeta, far far weaker rebels will defeat your gas team on defense. Gas loves his daze first on defense so telekinisis directly after his basic reduces cooldowns but the big deal versus most rebel counters is that with it the second pop ignores defense. You will manage fine controlling the character without it I'm sure versus most squads, but on defense without this zeta are sitting ducks to cls rebels.
Besides, short of galactic legends, GAS is the single most powerful character in the game. May as well make him as powerful as possible. Better than a zeta on Rose Tico ya know?