Multiple VIPS on GCP single nic F5

Good point, If no external firewall is found in front of the F5,  You can use instead of the Alias IP  - a forwarding rule.

https://cloud.google.com/load-balancing/docs/protocol-forwarding

The forwarding rule will listen for the second public IP and point the traffic to the F5 Compute engine (VM). On the F5 Device you should create a second virtual server with the destnation ip of the GCP public IP address. 

Let me know if it works for you.

Best Regards,