hundred cubic feet (Ccf)
literes
The measure is cubic feet.
That depends what aspect of the gas you want to measure: its volume, its pressure, its temperature, its density, etc.
Gallons or liters.
litres
Liter
litres
PPB
That depends what you want to measure about the gas: its volume, mass, transparency, temperature, etc. The really isn't such a thing as a "unit of measurement of gas", there are units of measurement for mass, volume, temperature, etc., all of which can be attributes of a specific gas.
You use litres to measure the capacity of a car's gas tank.
inside the gas tank, where almost all fuel sending units are --- after all, it does measure how much gas is in the tank doesn't it ?
As far as I am aware, CO is not a unit of measure.