CartesianProduct function from Combinatorica is for lists, and can be replicated using
If you are looking for the equivalent of
GraphProduct from Combinatorica, according to MathWorld that is
GraphComputation`GraphProduct[g1, g2, "Cartesian"]. I find it interesting to find a description of such an undocumented function on MathWorld.
Unfortunately it gives a graph with already set vertex coordinates, so the display may look very strange. I don't know how to remove vertex coordinates from a graph directly without recreating it from it edge and vertex list.