G is for testing Ground though there's honestly no point in grounding both sides of the cable. If you're making your own cables you would actually want to consider having only one side of the cable grounded for the sake of avoiding a ground loop, which causes a bigger headache than what ground tries to avoid.
Great tool. My only gripe is the 9V battery. It would have been great if the batteries were AA or something a bit more common. Not that 9V batteries are uncommon, but I certainly have more AAs at home than 9V batteries.