How to set default zone for Google Cloud project using gcloud command-line tool

Use this command to set the default zone for project myproject-123456 to europe-west4-a and the default region to europe-west4:

gcloud compute project-info add-metadata \
--metadata google-compute-default-region=europe-west4,google-compute-default-zone=europe-west4-a\
--project myproject-123456

Also see the official reference for more detailed information.