Dark matter substructures around nearby galaxies provide an interesting opportunity for confusion-free indirect detection of dark matter. We calculate the boost over a smooth background distribution of dark matter for gamma-ray emission from dark matter self-annihilations in tidal structures in M31, assuming a cross-section inversely proportional to the relative velocities of the dark matter particles as proposed by the Sommerfeld effect. The low velocity of the material in the structure results in a significant increase in gamma-ray emission compared to both the background halo and the predicted emission for a velocity-independent cross-section. We also calculate the expected signal for Fermi, for reasonable choices of the dark matter parameters. We find that for a cross-section proportional to v−2, the enhancement to the annihilation rate is sufficient to test the velocity dependence of the cross-section by spatial correlation with the stellar component of the M31 Giant Stream, given sufficient detector sensitivity.