file: /proc/sys/net/ipv4/conf/interface/drop_gratuitous_arp
variable: net.ipv4.conf.interface.drop_gratuitous_arp
Official reference

Drop all gratuitous ARP frames, for example if there’s a known good ARP proxy on the network and such frames need not be used (or in the case of 802.11, must not be used to prevent attacks.) Default: off (0)

Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.