With github you can search class name and it'll find any usages of it in your repo. Or basically any word and github will find usages of it.
Is there a way to do the same thing at gerrit?
Maybe there's a plugin or something?
Gerrit does not have any support to code search and AFAIK there isn't a plugin with such feature.
We use OpenGrok to accomplish this. It's a very nice tool.