Deploying Enterprise Grade Application on GKE

Good Morning  I am engaged in new project where decision has been made to host application on GKE.Web Application is being developed from scratch in Java ( it is not COTS product),I am part of Cloud Infra team and help App team with onboarding on GKE. I have my GKE Cluster prebuilt/Ready in Standard mode with  Nodes and Ingress/Egress setup and ready to deploys App/Microservices PODs.This is designed considering my enterprise HA/DR resiliency requirements.

At this moment I have no clue on workload requirement from sizing point of view and if application is CPU intensive or Memory Intensive. It would be Web Front, API Gateway ,Middleware and Backend Database( Cloud SQL PostgreSQL). App team will do the POC with minimal setup to start with .

However I would like to know as a Infra Solution Architect approach I should be taking and things I should be checking upfront at High Level to include in Solution design and DevOps team should refer this .

Any documentation/Training  on Design Enterprise Application on GCP, Best Practices ? I am going through "Reliable Google Cloud Infrastructure Design and Process Training" in the mean time .

Looking help in GKE area best practices like  1. Approach for deploying multiple environments (DEV,SIT,UAT,PRD)   2.  Setting up Pipelines 3. Sizing Guidelines 4.Building Container Images 5. Integration inside GKE Cluster

Please advice