You need to hack a few things in the code. I've wanted to do this myself for some time, so if you can hack it and submit a patch back, I'd appreciate it. :-)
Basically, idea goes like this:
Gerrit already has a daemon command that starts with "java -jar gerrit.war daemon". It fires up the SSHD, but no HTTP container or anything, and responds to commands on the SSHD just like Gerrit normally would. You could run this on the replication slave.
However.
This also fires up the submit queue during startup, and the mirror sync queue. These are started implicitly because Daemon.java calls GerritSshDaemon.startSshd, which in turn calls GerritServer.getInstance(), which has startQueues = true. On a slave we can't permit these queues to execute. You need to modify the code to ensure the queues aren't started from this daemon script.
The daemon also accepts uploads of new changes. You can't do that on the slave, as the master wouldn't have the data. So you need a command line option (e.g. "--slave") that tells it to either disable "git-receive-pack" and "gerrit-replicate" in the GerritCommandFactory class, or you need to provide stub replacements for these that know how to tunnel to the master. (If you do the latter than uploading to the slave just forwards to the correct server transparently, but its a lot more code to write, its easier to just deny the commands on the slave.)
You need to use GerritServer.properties to setup the database connection, and you would need to talk to the master database directly, which potentially means using SSL if you don't want the database password running in the clear on the wire. I have no clue how to setup PostgreSQL's SSL connection stuff. :-)
Actually, that's about it. The big thing is disabling the two things (queues and receive-pack/replicate commands) a slave can't do.
And yes, once the database is in git, the account data and ssh keys will be too, which makes it easier to replicate that to a slave. But we'd still need a "daemon --slave" flag to tell it not to permit writes here, because its only a mirror. Later, if we can get bi-directional replication working, we could lift that restriction, allowing those users to write locally, and let Gerrit replicate over the WAN.