Volume of one equivalent of a gas
The meaning of the word "equivalent" is relative to something. Fortunately, as a chemical term, that is still true. An equivalent is more completely called "a reaction equivalent" and is relative to the reaction. There are a few standard reactions (which are typically industry or discipline based) and it might be used usefully in limited (well constrained) situations. The problem is, especially when dealing with reactive gasses, that the words "uncontrolled" and "detonation" might also occur when explaining that you chose the wrong standard - the correct standard reaction is NOT always obvious or determinable in context. So, my best advice is that when someone uses "equivalent" ask what reaction, specifically, they are referring to. And when using the word yourself, always note the context. Examples: NCO equivalents, H(0) equivalents, H(+) equivalents may all refer to the same compound but would have different values.