Do heated seats use Gas?
 

seriously.

No. They use electricity.

The current draw on OEM seats is usually less than 10amp, often less than 5. My aftermarket heated seats draw only 12a, and that is at a significantly higher amount of heat than the OEM ones.

That current draw on the alternator will technically increase the parasitic loss on the engine, however only by a tiny fraction. You will not be able to measure the gas mileage difference. 1 single stop sign across a full tank of gas would probably affect your gas mileage more.