I'm creating my service in Google Cloud platform Kubernetes cluster, it is a simple application that has two endpoints one is public another is secured. For authentication, I use Cloud Identity. Cloud identity is using Firebase under the hood and JWT token has some attributes such as name which in my case is "firebase" and audiences which corresponds to my project id. For AppEngine app I can use cloud endpoints and it verifies JWT, but what I should use when deploying app to Kubernetes ? I saw that one option is Istio are there more options ?
You can use Cloud Endpoint with GKE.
Check this official tutorial : Getting started with Cloud Endpoints on GKE, it may help you.
For your information, you can also consider Cloud Run as a fully serverless alternative. It provides a built-in authentication layer.