Bleach novel vaguely describes how he created space and they ran with that to argue Galaxy-Universal Gremmy
Completely ignoring how it’s the same Gremmy who:
1: needed to double his power of imagination’s output to create a meteor only a few times bigger than Yhwach’s castle
2: crapped his pants at Kenpachi busting said meteor
3: was winded after his Galaxy Room and an exploding clone failed to take out Kenpachi
4: And outright hit the limitations of what his Imagination could conjure when he failed to both give himself Kenpachi’s power with a body able to contain it too
Imagine sucking so hard at reality warping that Kenpachi is too much for you to recreate