I think one can look at this through the "it's magic, silly" filter and perhaps glean some insight.
Regarding the gaseous form issue, one could imagine that glitterdust has its affect based upon life force or some such thing. In which case, all living things in its area of effect would be affected regardless of form, etc.
Remember, the glitterdust is a magical dust. Real dust wouldn't have all the effects of magical dust. It's not as simple as saying "dust doesn't affect incorporeal creatures" because it's not dust. It's magic dust.
Just a couple thoughts on the matter.