dbus-glib uses libdbus, GDBus doesn't. Instead, it relies on GIO streams as transport layer, and has its own implementation for the the D-Bus connection setup and authentication. Apart from using streams as transport, avoiding libdbus also lets GDBus avoid some thorny multithreading issues.

dbus-glib uses the GObject type system for method arguments and return values, including a homegrown container specialization mechanism. GDBus relies uses the GVariant type system which is explicitly designed to match D-Bus types.

The typical way to export an object in dbus-glib involves generating glue code from XML introspection data using dbus-binding-tool. GDBus does not (yet?) use code generation; you are expected to embed the introspection data in your application code.