Was asked to give my opinion, relating to the current stance of the thread.
The characters that would effectively scale with this feat or are above it, would be Shikai Zaraki, Adult toshiro, Post RG Bankai Byakuya, Post RG Bankai Renji, Vollstandig Uryu, Unsealed S0, Ichibei, all forms of Yhwach, Gerard Valkyrie, Ichigo, Aizen, Jugram and of course Gremmy himself.
An argument could be made for Yamamoto as well, but im not going to argue for that currently.
We've seen that Uryu > Unsealed Senjumaru, of which the latter already has a 4-A feat, by making the cosmology tremble by flexing her power.
Post RG Renji was capable of matching Uryu in combat, although being weaker obviously which still makes him scale.
Considering Byakuya has always been stronger than Renji, and underwent the same RG training, it's safe to assume that Byakuya > Renji still.
And all of them, along with Adult toshiro, and Shikai / Bankai Kenpachi were locked in combat with gerard, who scales above them.
All this does is simply prove that the potency of gremmy's creation feat in turn, fits perfectly with the current level of power, of the top tier characters in the verse, as the S0 feat was going to be used to further scale the top tier characters, that are relative, slightly weaker or above S0 in scaling.
This makes the feat consistent, with what the verse was going to be revised to.
The scaling chain itself should look something like this: Ichigo >= Yhwach > Aizen > Gerard > Ichibei > Uryu >= Jugram > Unsealed S0 (4-A) > Kenpachi > Adult toshiro > Post RG Byakuya > Post RG Renji > Gremmy (4-A)
Yet again, you could argue for Pernida and Yamamoto to be a part of it, but it doesn't really matter as the scale is not made to be 100% accurate.
Im just giving a general example of what the scaling should look like, to support the consistency of the feat that was shown.
As for the feat being 3-B, instead of 4-A, the minimal scaling would be 4-A which is what the OP is currently arguing for.
I would personally wait for any type of higher scaling arguments, until the 4th cour comes out so the scaling chain itself is further solidified.
Overall, i agree with the current proposition of using the meteor feat, for their base forms, and the outer space creation feat, for Shikai zaraki, and gremmy with his clones.
(The base form scaling should be an "atleast" imo)