ComplementGraph( gamma ) 
ComplementGraph( gamma, comploops )
This  function returns the complement of the graph gamma.  The optional
boolean  parameter comploops  determines whether or not  loops/nonloops
are   complemented   (default:   false   (loops/nonloops    are   not
complemented)).
    gap> ComplementGraph( NullGraph(SymmetricGroup(3)) );
    rec(
      isGraph := true,
      order := 3,
      group := Group( (1,3), (2,3) ),
      schreierVector := [ -1, 2, 1 ],
      adjacencies := [ [ 2, 3 ] ],
      representatives := [ 1 ],
      isSimple := true )
    gap> IsLoopy(last);
    false
    gap> IsLoopy(ComplementGraph(NullGraph(SymmetricGroup(3)),true));
    true 
GAP 3.4.4