Usually ARP (Address Resolution Protocol) is used to resolve IP address to MAC address. This is very common when ping is triggered. Source machine MAC layer has to send ping packet to correct destination machine for that it needs destination MAC address of destination IP. If destination machine is within the same sub net it will get the MAC address from switch/gateway and it will form the packet with destination MAC and send it to destination machine.
If destination IP is outside from the source machine network then source machine will send packet to gateway and then gateway will take care.
Gratuitous (Unnecessary) ARP is used by a Host to detects duplicate IP assignment within the network. It is a special kind of broadcast ARP with same source and destination IP. It is usually informs to other Hosts in the network that which host is owner of a particular IP.
In LTE when we are talking about connection less data (without GTP-U bearer) plane, at part of handover procedure Gratuitous ARP will be used by target eNodeB to update the other network elements saying I am the owner of this IP i.e. UE IP address.
This is what I know in the context of Gratuitous ARP and ARP.
