Create a service account for your GCP project as described here with the following roles (these roles could be more restrictive if required):
Kubernetes Engine Admin role.
Container Registry Service Agent role.
Storage Admin role.
Storage Object Admin role.
Generate a service account key for your service account as described here and export it as a JSON file.
GOOGLE_APPLICATION_CREDENTIALS variable and point it to the downloaded service account key from the previous step. For example: