GTK 4 is not available on any released Debian version. The author stated he's working on Debian, so there's no way he could choose GTK 4.
At the time the author started this port GTK4 likely hadn’t been released at all let alone have bindings for languages like Ada.
> let alone have bindings for languages like Ada.
Language bindings for any GObject powered are usually automatically generated from their introspection data (.gir) generated from g-ir-scanner.
So that wouldn't be the issue.
The problem is that GtkAda is called a thick binding which means that a lot of C-like code is refactored to Ada-style one.
So it's just not a simple generation.
Thick bindings aren't refactored code, they're a different interface. "Thin binding" means a shallow interface, possibly even autogenerated from C header files. "Thick binding" is usually abstracted, creating proper Ada data types (including the normal restrictions on them), different functions and procedures (perhaps bundling common sets of activities that would be a series of calls in the base C library), adding Ada tasks if its sensible to the system (which it may be, like wrapping around a network library, you can have the network connection handled in its own task).
I know but I didn't want to dive into details but you're right, this may need clarification
It’s also worth mentioning that gcc has a command-line flag -fdump-ada-spec that will generate an Ada spec (.ads file - Ada version of a .h) from a C header for easy binding with C libraries.