Is it safe to power an MQ-7 gas sensor with a 9V D.C supply?
You can, but you will have to reduce the voltage to the 5V the sensor needs, otherwise you will blow up the sensor.

To that end you require a voltage regulator. You can use a linear regulator like the Arduino has on-board, but they are very wasteful of power and will dissipate the excess 4V of electricity as heat.

Better would be to use a switching regulator (commonly and cheaply available on eBay and other sites) to step the voltage down to 5V more efficiently.