Hello,
GTK plus has no accessibility related support for windows. GTK
apps are only accessible on linux as far as I can tell.
Here is a GTK bug listing some things on how accessibility
support for GTK on windows should be developed however the
enhancement request is there for some ten years already I don't
expect anyone just magically doing it in the mean time.
Here it is so you can take a look on how it stands...
https://bugzilla.gnome.org/show_bug.cgi?id=303304
Greetings
Peter
