ResistanceMatrix[g_, prec_:$MachinePrecision]:= With[{m = PseudoInverse[N[KirchhoffMatrix[g], prec]]},
    Outer[Plus, Diagonal[m], Diagonal[m]] - m - Transpose[m]
]

rm = ResistanceMatrix[GridGraph[{10, 10}], 40];

rm[[12, 68]]