Hildon is an extension to Gtk, that adds Hildon specific widgets (like the pannable area). The majority of elements used in system applications are Hildon widgets, not plain Gtk widgets. Hildon also has many helper functions to use Gtk widgets in the Fremantle style, to apply the right themeing, etc. Qt for Fremantle is doing the same now, with custom widgets that behave like their Hildon counterparts. However, those haven't been used extensively in applications yet, as it was just released a very short time ago.