Comment # 6 on bug 1090532 from Frederic Crozat

(In reply to Michal Srb from comment #4)
> I would like to add that it is by design that no application can retrieve
> the content of screen under Wayland. It is up to the compositor to provide
> such screen sharing services or they won't be available at all. In this case
> the compositor comes from Gnome, so it is fully up to Gnome. This is a big
> downside of Wayland design.

this part is being worked on by upstream (in gnome-shell + pipewire + dbus api)
but it will take some time to be fully working and implements in things like
web browsers.

In the mean time, it is easier to either use a X11 session (selectable in gdm
session selector) or manually start GTK3 applications as X11, using
"GDK_BACKEND=x11 gedit" (for instance)