It depends a bit on the gas, for some common fuel gasses like Butane and Propane, the pressure in the cylinder is sufficient that the gas exists in equilibrium with a large volume of liquid.
The canonical way to measure fuel remaining in such a case is simply to weigh the bottle, as pressure will be determined mainly by temperature until all the liquid has been vaporized.
In some high burn rate applications you actually get a frost line on the outside of the bottle where the boiling liquid has absorbed so much heat that water freezes on the outside of the bottle at the level of the liquid fill.
Of course there are gasses that remain gaseous in the bottle, H2 for example, in which case a simple pressure gauge suffices.
Regards, Dan.