Unfortunately the 96Boards spec does not provide for any analog inputs (or outputs) on the boards. The only way is to either use a digital sensor, or perform Analog to digital conversion yourself. An example of an external ADC is this one: https://www.adafruit.com/products/1083. Alternately you can use a module that has the ADC included, such as this one: https://www.controleverything.com/content/Gas?sku=ADC121C_I2CGAS_MQ9
Remember whichever way you do it you will also need to level shift the I2C signals from the 96Board from 1.8V levels to the levels used by your sensor module (usually 3.3V or 5V).
Full Disclosure: I am an employee of Qualcomm Canada. Any opinions expressed in this or any other post may not reflect the opinions of my employer.