I can't speak for other platforms, but for GTK it's pretty obvious what is actually a GTK app, and what uses GTK as a wrapper. Sure, it might have the GTK titlebar and context menus. But all good GTK apps don't use a titlebar for the app title, they put widgets inside of it. They use libadwaita and get adaptive layouts for smaller app sizes.

Reusing native stuff is fine - it provides rendering, text, accessibility, window management, and other hard stuff for you. But it's not equivalent to a native app, because you'll never conform to each platform's UI guidelines without significant work per app.