Strips all the enemies armor, has the entire party and defense objectives take 0% damage, while applying a continous Gloom level slow to the enemies and popping out periodic energy orbs?
The funny thing about "Gara does Frost's job" is it unironically requires people to be bad at playing either frame. Frost's snowglobe is not his best ability (though nice when needed) and Gara's wall as a wall (and not a syngery bind with her 2) is probably her worst
Gara's 3 with the augment shoots out energy orbs way better than the Archon mod which has a long cooldown for only a chance of an energy orb.
Gara's 4 as a wall isn't that bad, you just need to touch a few enemies with it to get its HP up. Combine that with her taunting 3 and 50% dmg red 2 that can also heal for insane amounts with another 2 augments as well as give every enemy damage vulnerability and you have a frame that defends while supporting offense, and it works on Corpus and Infested too instead of just Grineer.
While, yes, it's probably not the most effective use of Gara, this second-rate Gara build is still better at defense than Frost. Frost is weak to melee Eximus, Gara isn't because her 3 is a taunting ability.
Gara is similar but doesn't do the same job.
She's a hybrid between Limbo and Frost in the regard that she offers damage reduction to both static and mobile targets while they give damage immunity to either static targets (Snow Globe) or mobile targets (Banish). The main difference is that Gara provides up to 50% DR to objects, whereas Snow Globe blocks everything.
In high level content, objects like Excavators don't have enough HP to survive even a few hits from the enemies even with Gara's DR and healing but Snow Globe can keep them alive by blocking the damage completely. There is Mass Vitrify, of course, but in order for it to work properly and have enough health, Gara has to hit multiple enemies while casting it. This means the enemies have to be close enough to her and hence to her defence target, which puts it at risk of getting destroyed before they get glassed.
Frost doesn't have this risk factor which is why he's better at defending.
Frost is better at scaling infinitely but Gara heals stupidly well nowadays with a couple augments and can keep the defense objective alive very well, especially considering that build also has the augment for applying her splinter storm to everybody. I would consider her superior for all normal gameplay including Steel Path, as she can handle melee Eximus significantly better than Frost. If even one melee Eximus slips through the cracks, the defense objective is a goner with Frost.
In addition, you can build range on Gara and easily always have an enemy in range to be hit with mass vitrify. On top of that, splintering storm protects against melee units and her 3 will taunt enemies into walking into the center (like say, where a defense objective is with a highly-damaging splintering storm). While an enemy being passed over by mass vitrify does mean that enemy is now inside the defensive ring, her 3 is an extra layer of protection that still works on Eximus and the enemy is slowed by mass vitrify's touch anyway.
In my opinion, Gara is easily superior at defense than Frost at any level, but that's harder to argue so I'll easily settle for anywhere outside endurance.