In the second movie, we see Gurren Lagann becoming something of an astral being thats able to reach huge heights they couldn't do before, like changing the probability of attacks working outright, or even actions themselves against them not working outright, being able to create anything out of thin air, being larger then our observable universe (was), and being able to survive the power of the big bang in it's weaker form.
Question is, how much more could Gurren Lagann evolve? How much more powerful could Gurren Lagann get?
Could they evolve endlessly and reach power levels rivalling someone like TOAA from Marvel Comics, or did they reach their peak?
The whole reason they got as powerful as they did was by evolving to meet that specific challenge.
They got beat down, they meet your power level, somehow, some way.
If you're a universal threat, they BECOME universal threat.
If you're a multiversal threat, they BECOME a multiversal threat.
If you're a multi-dimensional nigh-omnipotent threat, they BECOME a multi-dimensional nigh-omnipotent threat.
Thats why the anti-spiral couldn't defeat them, because in a way, they become as powerful as they need to, to reach whatever challenge. Because their spiral energy responds to their own will.
In effect, as long as they have the will to fight, they can keep creating new forms to match whatever opponent.
Not to say they could defeat ANYONE, but they are already way up in there in terms of power in ALL verses. There are less then a handful of people even from comics that could defeat STTGL, that doesn't mean its impossible to beat them, though.
For all we know their power could increase to the point they can make cows piss gold, Simon already borderline admitted to being able to bring people back from the f*cking dead.