(define-gboxed-variant-cstruct event "GdkEvent"
  (type event-type)
  (window (g:object window))
  (send-event (:boolean :int8))
            ...
            ((:proximity-in
              :proximity-out) event-proximity
             (time :uint32)
             (device (g:object device)))
  ... ))  

Proximity events are generated when using the wrapper for the XInput extension for GDK. The XInput extension is an add-on for standard X that allows you to use nonstandard devices such as graphics tablets. A proximity event indicates that the stylus has moved in or out of contact with the tablet, or perhaps that the finger of the user has moved in or out of contact with a touch screen.

This event type will be used pretty rarely. It only is important for XInput aware programs that are drawing their own cursor.