As described in this below Answers request...

https://answers.atlassian.com/questions/82988/branch-merging-with-gatekeeper-and-push-on-causes-git-failures-with-concurrent-builds

We are having the scenario where our build is long enough and we have enough teammates working on the same repo that branch merges being done by the gatekeeper process fail for not being at tip.

We have restricted the build to a single remote agent as a workaround. I would like to know if there is a configuration setting to toggle this behavior though.