Gerrit runs its ssh daemon on another port. If you run them both as the same user, it works fine.
We use gitosis currently for back-end management.
Gerrit does not add existing projects well. Pushing the kernel project into Gerrit causes one entry to approve per commit. That swamps the server. Gerrit does not have a way of handling rebases very well. (We have projects that have a regular consolidation on the end of the development trees.)
There are also some people (me for example) who loath the Repo command and prefer to work using git.
I am hoping we can migrate to Gitolite. I am going to set up a test server to see if I can identify problems.