If you are willing to go the microprocessor route, you could read the gauge with an ADC input, pass the signal through to the gauge using a DAC or digital potentiometer, and use whatever software algorithm you want in the middle. A simple version could just pause updating the output whenever an accelerometer reading is not close to zero, but you could then add timers, low-pass filters and more with only software changes.

These days the cost of parts (especially if ordered from overseas) is really reasonable. For some projects, you need expensive test equipment, but a multimeter (<$10 or sometimes free at Harbor Freight) would probably be enough. The learning curve might be the biggest obstacle.

 

If you do not do such ascents regularly, you might be negatively surprised about how much more your vehicle is consuming uphill compared to the usual consumption.

No surprise to me.
But I would still expect those who drive regularly in those conditions to know how much fuel is needed to reach their destination.

 

Some vehicles monitor the fuel level in the center of the tank so that the level stays fairly constant uphill, downhill, and on level road. And at least one model counted the strokes of a positive displacement pump to measure as the fuel was used. The system needed to be reset at each fill up. An actual in-line flow meter would allow measuring fuel consumed, so with a bit of math it would be clear how much was left.