What gaseous substances do humans emit?
The gases NO, H₂S, CO even have a function in the human body!

Nitric oxide is produced in endothel and neurons as messenger, and in macrophages as cause of nitrosative stress for imprisoned bacteria. Hydrogen sulfide is produced in cysteine catabolism and functions as messenger (only recently discovered). Carbon monoxide appears to act as messenger, too.

http://en.wikipedia.org/wiki/Biological_functions_of_nitric_oxide

http://en.wikipedia.org/wiki/Hydrogen_sulfide#Function_in_the_body

Wu, L; Wang, R (December 2005). "Carbon Monoxide: Endogenous Production, Physiological Functions, and Pharmacological Applications". Pharmacol Rev 57 (4): 585–630. doi:10.1124/pr.57.4.3. PMID 16382109