FortiGate can achieve HA using AWS ELB load-balancing. Two FortiGates can be deployed and associated with an ELB, and traffic is balanced between the two. If one FortiGate fails, the other handles traffic. External and internal ELBs are required if you want to serve both incoming and outgoing traffic for protected VMs.
Multiple AZs can be protected and turned high available depending on how you design the topology.
You can also combine AWS Route 53 to use DNS name together with ELB.