GTK 3 or 4? Xfce+MATE+Cinnamon are still popular enough options and I used the former for years. GTK itself isn't even a well defined target.

And GTK3 apps of which there are plenty do not appear native in a modern GNOME desktop much more so than a decent Qt app does.

The thing is that people in the real world talk about Linux support for an app, as in Desktop Linux support, and the only common denominator there for years was basically X11. I have heard exactly no one ever ask, oh when is that app going to be available for KDE or GNOME. For example, to take something recent here, do the people writing the Zed editor promise a particular DE support - no it's available for "Linux."

As to earlier mentioning distros - for a GUI app, there’s more pain from intra-distro variation than inter-distro, both across configurations and supported versions.

Now the big thing, the even bigger thing is probably native Wayland vs X11, but no one says, oh vendor will you please bring your app to KDE or GNOME, specifically. 99% of the time you are happy if you get anything at all. So the concept of native on Linux (Desktop Linux) just isn't something usually worth discussing.