The BMP180 is a digital device and connects via the I2C bus.
The MQ9 is an analogue device and produces a variable voltage. It can not be directly connected to the Pi.
You need to connect the MQ9 to an ADC (Analogue to Digital Converter) and connect the ADC to the Pi.
ADCs tend to have an I2C or a SPI interface. The choice is yours.
You can use a Arduino NANO for taking the MQ9 reading. Raspberry pi has no ADC inside so you have to get the value like this. Or use some sort of voltage detector(detects voltage and give us a certain value) that can digitise the values.