You might want to check out the main.c file in the client folder of the most recent Bluez source code. It's the source code for the bluetoothctl tool. Run it too. The source code shows exactly how they use GDBus, including proxies, agents, calling methods like described in the API documentation (/doc folder) and all that. It's in C and uses the high level API.
I suggest you step through the code because it took me 2 weeks endlessly trying to understand Bluez in C and the fact that there's no documentation, but when I read that main.c file I was ready in a day. Read up on proper DBus API documentation and more importantly the concepts. Some documents that helped me:
The gdbus tool: https://developer.gnome.org/gio/stable/gdbus.html
These contain all the calls to gdbus and objects in the main.c file and explain them very well. https://developer.gnome.org/gio/stable/gdbus-convenience.html
D-Feet, an invaluable tool to inspecting and learning about Dbus on your system. Try checking out the /bluez bus. https://wiki.gnome.org/action/show/Apps/DFeet?action=show&redirect=DFeet
or
sudo apt-get install d-feet
Not much of a tutorial, but worth a read to understand some concepts, as the bluetoothctl tool fits into what they're trying to say here. http://dbus.freedesktop.org/doc/dbus-tutorial.html
The bluetoothctl creates an interactive shell though, so it might not be wise to waste time trying to fit in your code, but just pick what you need from it.
