How to activate a GCP service account for other users in Linux.
First generate a key for the service account. Save as key.json.
Login to the server as that user and copy the key there. Activate the service account.
$ gcloud auth activate-service-account [ACCOUNT] --key-file=key.json
Once authenticated, you should be able to check if service account is active.
$ gcloud config list
A better option without needing a key.
gcloud config set core/account firstname.lastname@example.org