Gerrit and GitHub provide two different paradigms. If you want a straightforward answer, then yes Gerrit provides a number of extra features, but it isn't a replacement for GitHub.

Yeah, it does -- I'll let somebody who knows it better fill you in, but it really is night and day if you're serious about reviewing code. If you'd like to try a more powerful code review system than GitHub's to see what you're missing, without spending lots of time setting up and learning Gerrit, I'd recommend giving https://reviewable.io a whirl. (Disclosure: I built it.)