Gtk+3.0 bindings are being developed in the Lazarus-ccr
They are based on the headers from gtk+3.4.4 which can be downloaded here:
The current result can be found in the Lazarus-CCR in this svn address:
Or you can browse the files here
These files were generated with gir2pascal which should be updated to generate correct bindings if errors are found.
My simple program crashes immediately
You probably need to include the Math unit and put
Where is class X
Some classes have been deprecated in Gtk3 and the generator skips deprecated classes and functions.
Some functions are missing like g_signal_connect
Many functions are actually c macros and are unavaliable in the introspection files. In the case of g_signal_connect it is a macro around g_signal_connect_data.
Where are GtkHbox and GtkVBox
In Gtk 3 GtkHBox and GtkVBox are combined in GtkBox. In gtk_box_new you specifiy an orientation.