A wall-crossing formula for Gromov-Witten invariants under variation of git quotient

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.