For CI, you could have a git hook that could trigger a Jenkins build. For code review, you could use a tool like gerrit or phabricator or even set up an email list to send patch sets.

What other features do Gitlab, Github, or Bitbucket have that wouldn't be covered by that type of setup?

There are no bounds in engineering to how many wheels one can reinvent/reimplement. Ultimately, it's a matter of deciding where to invest the (limited) engineering resources.