NSX Advanced Load Balancer needs a GCP service account to authenticate access to GCP APIs.
To configure GCP API access:
If the Controller is running in GCP and a service account is attached to the Controller VM, select the option Use Controller Virtual Machine Service Account.
If the Controller is outside GCP (in some other public/private cloud) or there is no GCP service account attached to the Controller VM, add a user with GCP service account JSON key in NSX Advanced Load Balancer.