The two mounting holes are intended to be used with #2 USA size screws (not included).
Note that we also offer an alternate gas sensor breakout board from Sparkfun that is more compact, with a diameter equal to that of the smaller gas sensor and the broken-out pins beneath the sensor.
Connections
Connect VCC to 5 V, GND to ground, and OUT to an analog-to-digital converter (ADC). The board has two pairs through-hole pins for the required (but not included) sensitivity-selecting resistor RL, providing two mounting options - horizontal or vertical - between GND and OUT. The resistance of RL should be calculated using the datasheet for your particular gas sensor.
The MQ gas sensors have no polarity, so the gas sensor will work with the board all four ways it fits into the 6 pins (there two possible orientations on each side of the board).