  We prove a quantum version of Kalkman's wall-crossing formula comparing
Gromov-Witten invariants on geometric invariant theory (git) quotients related
by a change in polarization. The wall-crossing terms are gauged Gromov-Witten
invariants with smaller structure group. As an application, we show that the
graph Gromov-Witten potentials of quotients related by wall-crossings of
crepant type are equivalent up to a distribution in the quantum parameter that
is almost everywhere zero. This is a version of the crepant transformation
conjecture of Li-Ruan, Bryan-Graber, Coates-Ruan etc. in cases where the
crepant transformation is obtained by variation of git.
