Just to add more details. If you will try to configure VRRP on FortiGate that is already in HA Cluster, it will not work, because first that config will be copied to secondary device and second, when secondary device is passive, it will not have vrrp process running. Anyway, HA will provide you L3 redundancy. If primary device will go down, secondary device will be active and will be handling request as your gateway. Similarly, it is using virtual-macs to provide you this.
The concept of HA a-p is a system-wide of VRRP. While VRRP's scope focuses only on interface groups, HA does the fail-over system wide. With VRRP, its members communicate over the interfaces while HA communicates each others over heartbeat connections.