The zener diode is a good choice for something like this...and actually with a fuel gauge, you don't need an elephant gun to shoot a mouse. Just like Ohm's Law and other things electronic, explaining the zener can get complex, but here is a tutorial I think does a nice job of it:

https://www.electronics-tutorials.ws/diode/diode_7.html

Once established correctly a zener diode ought to last and work for a long time. And they're not expensive. I'll reserve comments about the other products out there, but honestly speaking...at least in my experience...the best, simplest and longest lasting (about 1 week past forever) 12 to 6 volt converter of all was the Vol-Ta-Drop. Unfortunately they stopped making them over 20 years ago. If you can find one, get it. Most people who still own one would be very hesitant to sell it.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end