If you want to verify for yourself that measuring battery voltage is not a "fuel gauge" do the following simple test.
1. Measure battery voltage and log. Say 6 volts.
2. Apply load and measure the time to reach some lower voltage, say 5 volts.
3. Remove load. Marvel as voltage returns to nearly the original voltage.
4. Apply load and measure the time to reach the the lower voltage.
5. Repeat.
Notice the battery voltage will return to nearly the original voltage even when almost dead. Notice the time interval to reach the lower voltage gets shorter every time you repeat the process. Battery voltage under load is an indication of the state of charge, but isn't a foolproof measurement.
A battery fuel gauge won't be fooled by a test like this, since it's actually measuring electrons into and out of the battery. It will give an extremely accurate indication of battery charge but it's very much overkill for this application.
Why does it matter if you call your simple test a battery fuel gauge? Imagine the poor SOB who is searching for info on battery fuel gauges and comes across this topic. He'll waste an inordinate amount of time going through what, 25 pages and counting, only to find there's nothing of the sort here.