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.
Works as advertised, and useful to make sure you've got solid connections on your punch down keystones. Similar items are widely available and often also have BNC coax testers on them. So I cracked it open, and all of the traces are there for the coax connector. The local electronics store had the 1N4148 diode and square LEDs needed, as well as the coax jack. Soldering was easy as the holes were push-through, and already had solder on them. To make the coax terminator with an LED, use the appropriate connector for the end of the cable being tested and simply connect the positive LED lead to the center wire and the negative to the outside. Once you punch out the extra hole on the front of the plastic case of the Master unit (below the G light) it will light up the new LED in sequence.
This item we have sold for many years. It's reasonable and is perfect for confirming RJ11/12 and RG45 cables are wired properly. We have sold probably 50 of them and they are pretty easy to use and always work. Great price too!
There are dozens of testers on the net that look just like this one, in various colors, but I've found them to be of low quality (a lot of three I bought from China didn't even test the ground connector even though there was a light for it). This tester is solid. It takes a stutter-step at the start of each test to make sure the testing starts from pin 1 instead of wherever the last test left off. You can even stop the test on a single conductor to perform "wiggle" tests. I wish the fast speed on the switch was all the way over instead of in the middle, but I can live with it.