It's Called Fog Panther. Flathub: FogPanther Claims to be written in GTK4. Looks like a modified GIMP. Made by a business consulting firm, looks sketchy to me.
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AFAIK GIMP only gained GTK 3 support on Nov 2024 and they're still working on GTK 4.
If this is really GTK 4, they might have forked from GIMP for a while now.
There's chances this really is a brand new UI on top of GIMP as opposed to just being a layout change and rename, but I guess someone would have to poke at it further to confirm.