The Gremlins "no feeding after midnight" rule makes perfect sense when you think about it.
Lots of animals can only breed or only feed during times in the day. If you were a Chinese mystic writing rules for an exotic species you'd write a rough guideline. And they clearly have a circadian rhythm tied to metamorphosis, and gizmo is being held in a delayed larva-type state. Perhaps he'd even grow into a huge mogwai is like one of those giant tadpoloes that never transforms into a frog.

What's more puzzling is clearly they're already primed for being nocturnal since sunlight hurts them. You get the idea they are cave dwellers, since the reaction is so strong. Perhaps the mogwais live around the cave entrance and eat bugs, then descend into the cave once they reach maturity and their hunger grows. Perhaps nocturnally killing and consuming what resides in the cave is a ritual to transform.