SUMMARY
GTK applications do not use the Global Menu widget in a Wayland session. Rather, they use their own GTK menubar.

STEPS TO REPRODUCE
1. Install community/appmenu-gtk-module
2. Add a Global Menu widget
3. Open GTK applications that use a menubar in a Plasma Wayland session. I have reproduced this with firefox-appmenu, qalculate-gtk, and pcmanfm.

OBSERVED RESULT
GTK applications use their own menubar, rather than the global menu. firefox-appmenu is a patched version of firefox that allows it to use appmenu, and it has native GTK menubar support removed. In the Wayland session, there is no access to the menubar at all. This is obviously because of the patch, and not related to the bug, but something observed nonetheless.

EXPECTED RESULT
GTK applications use the Global Menu as intended


SOFTWARE/OS VERSIONS
Operating System: Arch Linux (fully up to date as of Fri Oct 29 11:10:43 AM EDT 2021)
KDE Plasma Version: 5.23.2
KDE Frameworks Version: 5.87.0
Qt Version: 5.15.2
Kernel Version: 5.14.14-arch1-1 (64-bit)
Graphics Platform: Wayland
Processors: 12 × Intel® Core™ i7-9750H CPU @ 2.60GHz
Memory: 15.5 GiB of RAM
Graphics Processor: Mesa Intel® UHD Graphics 630