At work my team uses a mixture of CVS, SVN (and even folder compares!). None of which are completely satisfactory, although clearly SVN is the clear winner of that list. I had been wanting to experiment with a DVCS system - and was going to choose Mercurial because it had full http support and seemed less complex while giving most of the same functionality. I hesitated because Hg is really an application and not a library. Not sure why this really matters to me, but it does.
While taking a break from Hg, I read more on Git and discovered that Git and JGit both supported full http communications; Git didn't have this feature last time I had checked. I also discovered the JGit team implemented a servlet for serving Git repositories. When I learned those two things I knew I wanted to create a pure Java Git http server that was self-contained and included authentication & authorization. I was actually surprised that no one else had already done this; it seemed so obvious to me. :)
I read the ProGit book last Jan/Feb to get familiar with how Git was different than Hg and started working on Gitblit in the evenings in late Feb. I studied the Gitweb instance on kernel.org by navigating the available repositories and observing the urls. I created a facsimile of Gitweb/httpd/Git using Wicket/Jetty/JGit. Windows is my preferred deployment platform so its a first-class citizen in my project. I've been a Linux user for 15 years, but I am a terrible sysadmin. This is why I like Windows. :)
No company is supporting my project - I work on it in my spare time: nights and some weekends. I pushed it out as an Apache licensed project because I want others to be able to modify and integrate Gitblit into their own infrastructure as they see fit. There is nothing more irritating than having a cool open source project that you want to integrate but are handicapped by the project's license. I've gotten some contributions back through GitHub, which has been welcome. And the bug reports have identified problems which my dev team will not have to discover. :)
I do not know who is using Gitblit. I personally use it at work and most of the improvements are driven by "what can I do with Subversion that I can not (yet) do with Gitblit?". This is what inspired the federation capability (e.g. svn-sync). This is what is inspiring the planned Eclipse plugin for enumerating available repositories.
I'm aware of Gerrit. I have looked at EGit/JGit's Gerrit instance a few times. I do not use Gerrit myself; too much procedure for my small dev team. I see from the documentation that Gerrit supports Gitweb, CGit, and custom web ui integration. Substituting Gitblit for Gitweb should not be very difficult based on that information. I imagine Gitblit could also authenticate/authorize against Gerrit, although that is just a guess.
Of course now I'm motivated to build a useful AND popular open-source project. I monitor the download counts of the binary releases and I am looking forward to the next power of 10. :)