Conflicting information about the future of GtkBox
The documentation for Gtk3 HBox and VBox says that
You can use gtk_box_new() with GTK_ORIENTATION_VERTICAL instead, which is a quick and easy change. But the recommendation is to switch to GtkGrid, since GtkBox is going to go away eventually. See Migrating from other containers to GtkGrid.
This seems to indicate that, all things being equal, new code should prefer Gtk.Grid over Gtk.Box.
However, the documentation for Gtk.Grid says that
GtkGrid can be used like a GtkBox by just using gtk_container_add(), which will place children next to each other in the direction determined by the “orientation” property. However, if all you want is a single row or column, then GtkBox is the preferred widget.
If Gtk.Box is indeed going away, then the Gtk.Grid documentation shouldn't recommend using it.