If Gerrit rejects pushing a commit it is often the case that there is an issue with the commit message of the pushed commit. In this case the problem can often be resolved by fixing the commit message.
If the commit message of the last commit needs to be fixed you can simply amend the last commit (please find a detailed description in the Git documentation):
$ git commit --amend
If you need to fix the commit messages of several commits or of any commit other than the last one you have to do an interactive git rebase for the affected commits. While doing the interactive rebase you can e.g. choose 'reword' for those commits for which you want to fix the commit messages. For a detailed description of git rebase please check the Git documentation.
Please use interactive git rebase with care as it rewrites existing commits. Generally you should never rewrite commits that have already been submitted in Gerrit.
Sometimes commit hooks are used to automatically insert/update information in the commit message. If such information is missing in the commit message of existing commits (e.g. because the commit hook was only configured later) rewriting the commits will (re)execute the commit hook and so update the commit messages. If you do an interactive rebase to achieve this make sure that the affected commits are really rewritten, e.g. by choosing 'reword' for all these commits and then confirming all the commit messages. Just picking a commit may not rewrite it.
Part of Gerrit Error Messages