I couldn't find any program for Windows to send gratuitous ARPs, so I wrote a simple ARP sender.

Requires WinPcap to be installed. To build, expects the WinPcap dev pack to be at ..\WdpPack. The send_arp code is public domain, but limited by the WinPcap license.

Run without arguments to print out available devices.

Usage: send_arp <device> <src mac> <dst mac> <sender mac> <target mac> <sender ip> <target ip>

For example, to send a gratuitious ARP for 192.168.1.3 at address aa:bb:cc:dd:ee:ff:

send_arp \Device\NPF_{guid} aabbccddeeff ffffffffffff aabbccddeeff aabbccddeeff 192.168.1.3 192.168.1.3

You can download the binary here:
https://github.com/downloads/MichaelGG/send_arp/send_arp.exe