I’ve discussed traditional ARP in the past, in which one node requests the MAC address of another node. There’s also proxy ARP, in which one node responds to an ARP request on behalf of another node. This takes us to Gratuitous ARP, another kind of ARP.
GARP is an ARP Response that was not initiated by an ARP Request. A node uses the Gratuitous ARP as a broadcast to inform the whole network of changes to its IP to MAC mapping.
The utilization of a VIP in conjunction with network HA is a frequent use case for GARP. The backup node will broadcast a GARP to the network, telling all nodes of the VIPs mac address (itself) when the current node fails and takes over the active role.