Gdk offers some functions to manipulate the mouse pointer, but the one you'll be most interested is gdk_display_get_pointer
. There is also an example posted in this section of the forum that demonstrates moving mouse pointer using keyboard: http://www.gtkforums.com/about182.html
The other option would be to connect motion-notify-event
and obtain the x and y coordinates from second parameter of the function (event->x and event->y will be coordinates of th current location of the mouse pointer).
BTW, I think it would be better to post your question in GTK+ programming section.