So I have begun doing some research on the effects of idling. So my question is, does idling with the AC or heater on affect how much gas is given off? If possible, please give the percent of extra gas given off (e.g. AC causes 10% of extra gas to be given off). Thanks for your help!
In most cars (except for some of those sold in very cold countries, I believe) the heating works by using waste heat from the engine, so the only additional energy used for heating is that to run the fans. Conversely, air conditioning requires energy to operate. Therefore, using AC will usually use more energy than using heating.
Translating this to answer your question is more complicated. I assume by "how much gas is given off" you mean exhaust gas. Since an engine that is idling is running inefficiently at very low load, I'm not sure whether adding the load of an air conditioner would necessarily increase its fuel consumption, or exhaust output, significantly. Perhaps somebody with a better understanding or car engines could elaborate!
However, if there is a noticeable effect, then that effect will be greater from A/C than from heating.
Definitely air conditioning requires more energy, since heat is, as correctly stated in the other answer "basically free" because it's the waste heat from your engine.
According to this study by R. Farrington and J. Rugh of the National Renewable Energy Laboratory (NREL),
Current air-conditioning systems can reduce the fuel economy of high fuel-economy vehicles by about 50% and reduce the fuel economy of today ís mid-sized vehicles by more than 20% while increasing NOx by nearly 80% and CO by 70%.
This study was done in 2000, but is probably not that far off more recent figures, and it also shows that the cost is not merely in the energy used but in the increased pollution that results.
Heat is basically free: your car's heating system works by diverting some of the engine coolant through the heater block rather than the radiator, with the result that waste heat gets dumped into the cabin rather than the outside world.
In contrast, air-conditioning works by connecting the AC's compressor to the engine output, drawing off energy that would otherwise go to the wheels (and potentially requiring a higher idle fuel flow to prevent stalling).