  We present numerical simulations of disc-planet interactions where the planet
opens a gravitationally unstable gap in an otherwise gravitationally stable
disc. In our disc models, where the outer gap edge can be unstable to global
spiral modes, we find that as we increase the surface density scale the gap
becomes more unstable and the planet migrates outwards more rapidly. We show
that the positive torque is provided by material brought into the planet's
coorbital region by the spiral arms. This material is expected to execute
horseshoe turns upon approaching the planet and hence torque it. Our results
suggest that standard type II migration, applicable to giant planets in
non-self-gravitating viscous discs, is likely to be significantly modified in
massive discs when gravitational instabilities associated with the gap occur.
