In some cases you might want to use custom boot image instead of the one provided by Fortinet. One of these reasons is using GVNIC driver. By default images for Intel-based architecture (amd64) deploy instances with virtio driver. If you want to change it you need to create a private image with GVNIC support (FortiGate firmware already supports it, but it's not indicated in the image metadata). The easiest way to create a custom image is to copy existing public image with new options.
You can instruct the module to deploy using a custom image by providing image_name and image_project variables. GVNIC driver needs to be selected using nic_type variable. E.g.:
image_name = "my-fgt-image-name"
image_project = "my-project-name"
nic_type = "GVNIC"
Example code in main.tf creates a new image from existing public FortiGate 7.2.4 BYOL image and deploys a cluster with GVNIC driver using the custom image.