Just to clear up any confusion based on how you asked this question - Kalithea _is_ "an existing solution." It is not something new, but has been around since at least 2014. This is about the same time Gitea's predecessor, Gogs, was created. It's also around the same time Gitlab was launched.
I am less sure about when Gerrit was created, but it is possibly older.
I am less sure about when Gerrit was created, but it is possibly older.