Gas from decaying food is produced by microorganisms such as bacteria and fungi. These microscopic animals and plants "grow" on old food causing it to decay. In the process of their growth they digest the food by using enzymes that break the molecules of the food into bits that the bacteria (or other plants and animals) can use to live. When they break a big molecule (which for food is mostly made of carbon, oxygen, hydrogen, and nitrogen as well as small amounts of other elements) into little pieces, quite often the tiniest pieces emerge as gasses. The most common gasses produced by microorganisms are hydrogen, methane, and carbon dioxide. Very small amounts of other gasses may be produced. I tried to find resources but could not find very many. I suggest you try searches in www.google.com or www.wikipedia.org on words like: decay food gas, decomposition, rotting.

Better yet, visit a library and look for a book on microorganisms and decay.