The central concepts of D-Bus are modelled in a very similar way in dbus-glib and GDBus. Both have a objects representing connections, proxies and method invocations. But there are some important differences:
dbus-glib uses the libdbus reference implementation, GDBus doesn't. Instead, it relies on GIO streams as transport layer, and has its own implementation for 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 on the GVariant type system which is explicitly designed to match D-Bus types.
dbus-glib models only D-Bus interfaces and does not provide any types for objects. GDBus models both D-Bus interfaces (via the GDBusInterface, GDBusProxy and GDBusInterfaceSkeleton types) and objects (via the GDBusObject, GDBusObjectSkeleton and GDBusObjectProxy types).
GDBus includes native support for the org.freedesktop.DBus.Properties (via the GDBusProxy type) and org.freedesktop.DBus.ObjectManager D-Bus interfaces, dbus-glib doesn't.
The typical way to export an object in dbus-glib involves generating glue code from XML introspection data using dbus-binding-tool. GDBus provides a similar tool called gdbus-codegen that can also generate Docbook D-Bus interface documentation.
dbus-glib doesn't provide any convenience API for owning and watching bus names, GDBus provides the
g_bus_watch_name()family of convenience functions.
GDBus provides API to parse, generate and work with Introspection XML, dbus-glib doesn't.
GTestDBus provides API to create isolated unit tests GDBus Test Scaffolding.