Gratuitous ARP is an ARP packet for requesting own IP address.
Original ARP is used to resolve MAC addresses corresponding to IP addresses. What's the point of requesting your own IP address?
Gratuitous means "free of charge; out of favor; gratuitous; needless."
Is it really pointless?
The first use is to detect duplication of IP addresses due to misconfiguration, etc.
Under normal circumstances, even if you send an ARP request for your own IP address, there should be no reply from other terminals.
But what if you got a reply?
You can see that the IP address is already in use by another device.
With this mechanism, it is possible to check whether IP addresses are duplicated within the same network.