We've also been using OPA in production for use cases such as:
1. microservice authorization policy
2. internal webapp authorization policies via Envoy filter
3. kafka authorization

We also spoke at Kubecon 2019 about some of our use cases, you can check it out here: https://www.youtube.com/watch?v=LhgxFICWsA8

Gatekeeper / K8S admission is actually one of the main use cases we still haven't fully integrated (in the works though)!