Maybe a hint?

GTK3 and onward made a fairly significant design pivots in places IIRC. To the project's credit, they have documentation covering both migrations, but you can see it is not a "bump the version number and swat at any deprecation warnings" kind of weekend task if your application did anything nontrivial:

https://docs.gtk.org/gtk3/migrating-2to3.html
https://docs.gtk.org/gtk4/migrating-3to4.html

...along with that, it's more or less impossible *not* to change the UX in the process. Not just from theme breakage (that ship's largely sailed, unfortunately), but from UI patterns (like how the modern GTK/GNOME world likes putting primary buttons in titlebars, and will start doing so if you use the stock file chooser).