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.
This tester is an excellent value and ridiculously easy to use. I would definitely recommend it to anyone wishing to eliminate wasted time and frustration on a cable that may or may not be damaged. It's absolutely a great product and value. At my work we have a 250' ethernet cable that stretches from one side of the office to the other. We were concerned that it might have been damaged. I hooked up the remote portion of this tester to one end of the cable, hooked up the control portion to the other end, and instanly discovered that my cable was fine. What an awesome tool.
I didn't think this through, but I expected it to tell me if the patch cable I assembled was good. It only tells you if you have a signal on all 8 wires. It's still a good start since validating that the colors are in the proper order is easy.
Worked for me on a 100ft Cat5 run where my goal was to rule out the cable. Allowed me to easily identify the run, too. Not 5 stars because the documentation is fairly cryptic, especially for folks like me who have never used a cable tester and because the device requires one 9V alkaline battery which is not mentioned anywhere — I had to open the battery chamber to determine the battery.