The documentation page for GraphPlot seems to show that it uses Rule to identify edges. It also seems to show that each Rule indicates an edge in one direction. Thus if you want an undirected edge you give one rule for each direction. Does this work for you?
GraphPlot[{1 -> 2, 2 -> 1, 3 -> 1, 3 -> 2}, DirectedEdges -> True, VertexLabeling -> True]
Click on the Details and Options on the documentation page to see more information on how to use GraphPlot.