The MQ gas sensors are electro-chemical with a heater.http://en.wikipedia.org/wiki/Electrochemical_gas_sensor
They are sensitive for a range of gasses, and if one of those gasses is in the detector, the conductivity between 'A' and 'B' gets better.
With a load resistor, the voltage over the load resistor changes, because of the better conductivity between 'A' and 'B'.
You can try to calibrate them for a certain amount of gass, some do even calculations with the value to get a percentage. But that is just a wild guess, it is not accurate at all.