sourceTags property

If source tags are specified, the firewall rule applies only to traffic with source IPs that match the primary network interfaces of VM instances that have the tag and are in the same VPC network.

Source tags cannot be used to control traffic to an instance's external IP address, it only applies to traffic between instances in the same virtual network. Because tags are associated with instances, not IP addresses. One or both of sourceRanges and sourceTags may be set. If both fields are set, the firewall applies to traffic that has a source IP address within sourceRanges OR a source IP from a resource with a matching tag listed in the sourceTags field. The connection does not need to match both fields for the firewall to apply.