You can deploy FortiGate for Google Cloud as VM instances. Supported machine types may change without notice. Currently FortiGate supports standard machine types, high-memory machine types, and high-CPU machine types with minimum 1 vCPU and 3.75 GB of RAM and maximum 96 vCPUs and 624 GB of RAM in the predefined machine type lineup. You can also customize the combination of vCPU and RAM sizes within this range. See here for more details on predefined machine types.
Latest supported machine types can be seen under machine type selection if you try to launch FortiGate from the marketplace listing or Compute Engine portal.
FortiOS supports hot-adding vCPU and RAM. However, GCP may not support this. See Changing the machine type of a VM instance.