If you look at the freeze output, you'll notice that _gtk is
not linked into your application: It is a shared library (.so),
so it can't be frozen.
Instead, the resulting binary will look for _gtk.so on sys.path.
Try copying _gtk.so into the directory where the binary is,
or set PYTHONPATH before running the binary.
If you want to freeze _gtk into the application, you need it as
a static library.