IsConnectedGraph( gamma )
This boolean function returns true if and only if gamma is (strongly)
connected, i.e. if there is a (directed) path from x to y for every
pair of vertices x,y of gamma.
    gap> IsConnectedGraph( JohnsonGraph(4,2) );
    true
    gap> IsConnectedGraph( NullGraph(SymmetricGroup(4)) );
    false 
GAP 3.4.4