> Old MacOS 10.5-10.9 do not work, no windows opens (10.7 used to work, broke) so I cannot use these as comparisons
X11 is not the most robust thing, to put it mildly, but I would expect gtk3 app to work with GUI, as long as gtk3 works on a given system. (gtk4 is bad, so there anything can be broken randomly.) I can try out of curiosity and see if it gonna work on powerpc.
