analog level conversion in ADC circuit
You don't give voltage at pin GAS_V ,this is your output pin, you read value from it. So there is no need to use voltage divider at this node.The way you are using voltage divider it will give you a fixed 3.3V at that pin which may damage your sensor.

According to your datasheet your supply voltage must be 5V (so if you are thinking about using 3.3V here, change you sensor that operate in 3.3V)and your output voltage will very from 2.5V to 4V.I'm guessing you want to bring this output range within 3.3V. 

If you want that , use GAS_V pin as the supply pin for your voltage divider and take the divided voltage as input for ADC. consider GAS_V as 4V (maximum value) and calculate values of resistances that give you 3.3V .

A fair warning , your divided voltage may not 100% correct all the time with good precision as we are not considering input resistance and max input current of the device you are using to read ADC value. If it's input value is too high comparing your voltage divider resistances, it will work fine. If not then you can design it considering input resistance of the device. Fist see the datasheet of the device to find input resistance  and current of the ADC pin and consider this resistance is connected in  parallel  with bottom resistance.