[Pharo-users] How to use Spec 2.0 with GTK in Pharo 9.

Hi. I'm learning Spec 2.0 with the help of the book "Building user interface with Spec 2.0". In section 2.17 of the book, the authors make a subclass of SpGtkConfiguration. However, no such class exists, only a SpMorphicConfiguration. Moreover, if I do something like: "app useBackend: #Gtk", I get a NotFound error. Again, "app useBackend: #Morphic" work as intended. As Pharo 9 annonced the availability of a GTK backend, I would like to know how to access it. Thanks. tau.