Pygobject does not “implement” libraries: you use pygobject to access libraries through the introspection layer.
Pretty sure you can already do:
from gi.repository import Json
and start using JSON-GLib.
Of course, I’d recommend using Python’s json module, unless you’re trying to integrate with object serialization. Even in that case, you cannot serialize a GtkTreeStore instance automatically: you have to iterate over the store and build a JSON array. At that point, you don’t need JSON-GLib.