Namespace GTK not available

[Degamisu]

$ python nrsc5_gui.py
Traceback (most recent call last):
  File "C:/users/USER/documents/radio/tools/nrsc5-gui-2.0.0/nrsc5-gui-2.0.0/nrsc5_gui.py", line 35, in <module>
    gi.require_version("Gtk", "3.0")
  File "C:/msys64/mingw64/lib/python3.11/site-packages/gi/__init__.py", line 122, in require_version
    raise ValueError('Namespace %s not available' % namespace)
ValueError: Namespace Gtk not available

I'm using Windows 11 + MinGW64 (MYSYS2)

This occurred after build and I cannot find a fix to it. All packages are installed, including a fresh install of GTK.