  We perform a Feynman diagram calculation of the two-loop scattering amplitude
for gravitationally interacting massive particles in the classical limit.
Conveniently, we are able to sidestep the most taxing diagrams by exploiting
the test-particle limit in which the system is fully characterized by a
particle propagating in a Schwarzschild spacetime. We assume a general choice
of graviton field basis and gauge fixing that contains as a subset the
well-known deDonder gauge and its various cousins. As a highly nontrivial
consistency check, all gauge parameters evaporate from the final answer.
Moreover, our result exactly matches that of Bern et al., here verified up to
sixth post-Newtonian order while also reproducing the same unique velocity
resummation at third post-Minkowksian order.
