I have 2 namespaces in my kubernetes cluster: development and production. I'm currently adding a third namespace: staging.
I'm NOT using terraform with which this task would have been supposedly simpler.
I'm looking for solutions within the GCP ecosystem to provision a workload in the staging namespace with all the environment variables and configurations of the development namespace.
Please check my answer to similar question here. Unfortunately there is no ready solution for that within GCP, especially if you want to migrate workload from the existing cluster between different namespaces. However you can use for that purpose Heptio Velero. It's nicely described in this article.