As for the permission, as clever as it is the plugin is unmaintained (no matter what the MAINTAINERS file states, Alexander is gone as of the latest news I had) anyway, so it's up for grabs :)
Wasn't aware of that, so yeah, its up to the contributor.
As for GTK4, thats a whole other kettle of poison `</deliberate misspelling of poisson>`, but until Geany looks at it, its not worth thinking about for an unmaintained plugin :)