Your Arduino will read the voltage of the output of the sensor. analogRead() returns a number from 0 to 1023, which represents 0 volts to the voltage of your Arduino (usually 5 or 3.3 volts.) You can calibrate the output against known concentrations and do an analysis, but you cannot determine the sensor resistance this way.
If you wish to do so, construct a voltage divider, with the sensor on one side, and a precision resistor on the other. Read the voltage from the center and calculate the resistance from the voltage divider formula.