Gitbuilders were VMs (QEMU or LXC) typically found on the irvingi or senta nodes. They ran autobuild-ceph and were maintained using fabric.
They're in the process of being phased out in favor of CI infra being built by Alfredo and Andrew so I won't go into a ton of detail on them.
Assuming fabric actually completes successfully, you'll need to find the sha1 of the master commit (check the gitbuilder's http page and click master in the top bar) and rm it from /srv/autobuild-ceph/gitbuilder.git/out/pass/$SHA1 to force a build on it.
Also chown autobuild-ceph:autobuild-ceph /srv/autobuild-ceph/logs/*
Copy /srv/gnupg/{pubring.gpg,secring.gpg} from another gitbuilder to /srv/gnupg.
Grab /srv/autobuild-ceph/{rsync-key,rsync-key.pub} from a newer gitbuilder (gitbuilder-kernel-deb-xenial-amd64-basic.front.sepia.ceph.com has them). Just make sure it's RSA.
chown -R autobuild-ceph:autobuild-ceph /srv/gnupg chmod 600 /srv/gnupg/*
apt-get install libssl-dev bc
Also had to comment libunwind7-dev from fabfile.py.