usually when it comes to graphic design work (i’m an open source graphic designer and mayhaps even programmer but) i use inkscape of course, and gimp totally. i’ve gotten very good and learned so many stuff, but something i enjoy about inkscape and gimp is the fact that they still use ctrl-z for undoing and ctrl-y for doing again!!! that’s classic and very linux-like and is pretty much the software saying “we will NOT accommodate our keyboard shortcuts to make our software feel LIKE adobe products”, because adobe and all proprietary graphic design software uses ctrl-z and ctrl-shit-z for undoing and doing, but linux prevails and keeps ctrl-y.
when i was drawing a bit of stuff in krita, i found out that the shortcut for undoing was ctrl-shift-z, i know why this was made, because they want to accommodate the adobe people, the ones that like windows and proprietary software, they want to appeal to proprietary companies, that shortcut showed me that. and that is why i feel like krita is trying to be an open source software dressed as a proprietary software.
it’s like when that american german spy asked for 3 glasses of liquid but held his fingers in the “not german” way.
I think the ctrl-y vs cmd-shift-z was a Windows vs Mac thing. A lot of commercial gui software originated on Mac including Photoshop (and much of Microsoft Office) and Mac remains popular with the creative crowd. Older Linux gui software used to be weird, either cde/motif stuff or things that looked like they were developed on an Amiga. Keyboard standardization was never a thing with linux - eg emacs and vi.
I believe ctrl-shift-z is standard across many Gnome and KDE apps now. All the ones I could quickly test anyway. Inkscape and Gimp kind of do their own thing but Inkscape definately has ctrl-shift-z showing as the primary redo shortcut for me although it seems to support control y as well. So I think Gimp is just weird as usual. The UI doesn’t conform to the expectations of contemporary Linux users let alone people from other platforms. I would probably just assume Gimp was broken, close it and open Krita instead.