The status is that the refcounting parts probably still need some work, but most things work, including, IIRC, inheriting a GI class from a Pike class and overriding the methods in it.
I've demoed it with instantiating a web browser widget in a GTK3 window, but due to the lack of Cairo no custom widgets are possible.
The branch is off early 8.0, but if there is new interrest in it I can probably bring it up to date for 8.1.