Some applications can do that ─ Chromium is one of them, and it doesn’t even need appmenu-gtk-module for that ─ but not all of them do. Pamac does not, and neither does TimeShift.
This here-below is Claws Mail, a GTK-based email and Usenet client ─ I am using it for Usenet only ─ and clearly it works, albeit ─ as I said already ─ that in Plasma 5.19.4, you have to activate a KDE/Qt-based application first before the GTK application’s menu is visible in the Plasma global menu widget. 
And this here is GIMP.
