Can I also evaluate input against my OPA policies with this?

As OPA is now a graduated project in CNCF, this will definitely make things easier for the future as the need for policy-as-code in the cloud seems to be increasing significantly.

So cool to see work on a common language and infrastructure for solving authorization problems. I feel like we've all reinvented this wheel dozens of times in our careers.

Is there GitLab CI support, similar to the GitHub actions?

This is a great tool - using it successfully to package up OPA policies for Aserto.