See Google Cloud Marketplace.

Deploy a FortiGate-VM instance on Google Cloud Compute Engine from the custom image without using the Google Cloud Platform marketplace. See Google Cloud Compute Engine. You must deploy FortiGate in this method when:

• FortiGate is required to be deployed inline across multiple networks and multiple network interfaces must be assigned to the instance. The FortiGate marketplace launcher does not support assigning multiple network interfaces to a FortiGate instance. (A future release may support this). Google Cloud also does not allow changing the number of network interfaces after deploying VM instances.
• You do not want to use the Google marketplace launcher. For example, you may want to use this deployment method if your organization does not allow you to browse marketplace websites in its IT policy.

Deploy a FortiGate-VM (BYOL) instance by using the Google Cloud SDK on your local PC. This is a method of deploying FortiGate-VM on GCP outside of the marketplace product listing and without creating an instance on the Google Cloud Compute Portal. This method also allows assigning multiple network interfaces to the VM instance. See Google Cloud SDK.