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
litres
Liter
You use litres to measure the capacity of a car's gas tank.
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.
No, Torr is a unit of pressure, not energy. It is used to measure gas pressure in many industries, such as chemistry and physics.
Natural gas is typically measured in cubic feet (ft³) or cubic meters (m³). Another common unit of measurement is British thermal units (BTUs), which is a measure of the heat energy produced by burning natural gas.