GitLab Community Edition is MIT licensed. I don't know that it gets much more open source than that. I've survived at very very large companies on CE, never needing any of the paid features.
But if you're relying on that feature rather than just agreeing, training, and trust, then I'm also not sure that's the best way to go about it. I'd personally much rather be somewhere where the rule is flexible and can be left to the MR submitter to decide when it's important to have more eyes on a change.