Eh, GTK as a whole is… well, not
healthy for cross-platform development. It used to be a good deal better (though it was never really
excellent), but from GNOME 3 they’ve pretty much decided to flat-out ignore other platforms and their conventions, and shift GTK from being GNOME-focused but fairly platform agnostic with pretty good look-and-feel theming to being just GNOME GNOME GNOME and wildly out of place in look and feel everywhere else.
Of course, other options in a similar space like Qt aren’t entirely better in that they don’t behave fully like GNOME on GNOME platforms. The best cross-platform native-feel apps, like Firefox, tend to use GTK on Linux (or possibly KDE/Qt in KDE?) and other things on other platforms.
As a Linux user I inevitably use quite a bit of GTK software (though barely any GNOME stuff, if you savvy the distinction I’m drawing), but I dislike almost all of the directions it’s headed in since GNOME 3. Decent scaling is one of the few redeeming features.
sprash is describing a very—ahh—ideologically pure position, but I do deeply sympathise with a desire for a return to the GTK+ 2 ways. My ideal would probably be to scrap GTK+ 3 and 4 and create 5 derived mostly from GTK+ 2’s philosophies, just including the good technical parts of 3 and 4, but determinedly not a full style unto itself. Also I’m fed up with client-side decorations matter and want better support for server-side decorations, as was done traditionally and as is still generally preferable on Windows and in most Linux window managers, or even platform-matching client-side decorations, which would be good for macOS and Windows and frankly GNOME.
GNOME has basically sabotaged GTK as a cross-platform library—even as a Linux-wide thing (apart from KDE). They’ve taken it over to serve their own purposes and vision. Their vision does improve and unify some parts of GNOME desktops, but as a user of something other than GNOME (Sway), I don’t like most of what they’ve done to GTK.