Please understand what I am about to say is all conjecture. As I understand it, the temp gauge and gas gauge are really just volt meters. You will note that both gauges have little dashes at 12 o'clook. I understand those are calibration marks. I am guessing that is where the needle should be when 6 volts (half of 12) is applies across the terminals. (If only one terminal, the mounting stud is also an instrument case ground, or negative, terminal). I am guessing the accuracy of the voltmeter movement is pretty poor so they just "tune" them to the mid-point of the scale, for obvious reasons.
If they are like the old style volt meters I grew up with, there should be a variable resistor (potentiometer) somewhere inside. Most likely it would look like something with a slotted screw driver tab in the middle. By inserting a slotted screw driver tip in that slot and turning it one way or the other, you vary the resistance, which affects where the needle reads.
Again, I have never opened up one of these so I am just guessing. If you want to have a look, proceed at your own risk. If you do and find what I am describing, I would say hook up a 6 volt battery as you did the 9 volt one with the gauge on a workbench. Rotate that potentiometer a little from side to side and try to get it to read at that 12 o'clock dash mark, remove battery, reassemble gauge, and install.
CAUTION, that potentiometer may be frozen in place after all these years and you may not be able to move it or end up breaking it trying. Patience here is a virtue. If it will not move under gentle persuasion, disconnect the battery and perhaps discretely apply a bit of PB Blaster or similar product to free it up after "soaking" for a while. Soak up any spilled liquid off the circuit board with a q-tip. Not sure if what you would be using is electrically conductive or not and we do not want any shorts! As I said this is all conjecture, but you could at least have a look to see. It is what I would do but that is me not necessarily you. My 2 cents. Cheers, Gene