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.
I make up my own RJ12 straight-through and RJ45 crossover and straight-through cables for model railroading. This tester is simple but effective in this role. Verifies all conductors as continuous (or not) and that they are of the correct pin assignment (straight or crossover). You have to know what a crossover pin out should be, but a quick cheat-sheet and eventually your memory will be enough. For simple installations I can recommend this tester.
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.