I know I am not the only one having done so: create a template sensor with device_class: battery just to have an icon nicely showing a level, but for a measure having nothing to do with a battery.
We could add to HA a device_class simply called ‘gauge’ that would take benefit of the different Material Design icons for gauge:
The icon displayed for the related sensor would be selected using the same logic as for the battery device_class, e.g. more than 75%, gauge-full; more than 50%, gauge; more than 25%, gauge-low; else, gauge-empty.