Does it make sense for non-Gnome applications?
I strongest beleave that a lot of things in Gtk, have not sense currently from outside Gnome because that concepts are don’t used outside GNOME at all and in my personal opinion this will make your application fully out of place in other platforms.
I think that the intention of all new design patterns of Gtk is make it as an standard also outside GNOME, but in practice i think this is impossible. You also will need to take in account that more new design patterns can come later to Gtk, so you will have more things to worried about than just Menus if you select Gtk as a toolkit to your app, please see: GTK support for GNOME design patterns
If you have a plan of migrate your application from Gtk3 to Gtk4 in the future is another thing to worried about. Please also see that they have the idea of discontinue the usage of GtkMenus in the future and then force you to select the new GMenuModel. With other features I can not ensure that will be different.
To complicate even more the things there are currently no official way to modify the look and feel of a Gtk4 application outside GNOME as was occurred with Gtk3 in Unity desktop, for example. See the related issue here.
So, a Gtk application will look like more and more out of the place in other desktop environments and that means to me that the GNOME pattern of Gtk is a wrong selection if you want to have an application that:
work and look properly on various window managers and OS’es