By finding various GTK3 code snippets on the internet I have learned that for a GtkMenuItem the callback for the "activate" signal is
void Callback1(GtkMenuItem *menuItem, gpointer userData);
and for a GtkAboutDialog the callback for the "response" signal is
void Callback2(GtkDialog *dialog, gint responseId, gpointer userData);
Where are these and the other callback signatures documented? I have looked in the reference manuals but cannot find documentation about this.
https://docs.gtk.org/gtk3/
https://docs.gtk.org/gobject/
https://docs.gtk.org/gio/
Go to the documentation website, search for GtkAboutDialog
( https://docs.gtk.org/gtk3/class.AboutDialog.html ).
On the right side near the top, select Signals to go to the signals section. The response signal is inherited from the GtkDialog class. Open the GtkDialog section, the GtkDialog::response signal is listed there.
Select the GtkDialog::response link, and the signature for the response signal will be listed.
You can also search for response. In the results list, only look at the items marked as Signal to find the correct match.