gobject:object-pointer (object)

object -- a g:object instance
pointer -- a foreign pointer to the C instance of object

The accessor for the pointer slot of the g:object class returns the foreign C pointer of a g:object instance.

(setq label (make-instance 'gtk:label)) => #<GTK:LABEL {E2DB181}>
(g:object-pointer label) => #.(SB-SYS:INT-SAP #X081BDAE0)