Gas from bacteria that's not methane
Microbes can produce several gasses other than methane.  


All microbes produce $CO_2$ through the oxidation of reduced carbon


Additionally some metabolic pathways produce other gases.


Photosynthetic microbes produce $O_2$
Sulfur reducing bacteria can produce $H_2S$ (as @ohcanada points out)
Denitrifying bacteria produce $NO$, $N_2O$, and $N_2$
Fermenters produce $H_2$ (as @mart indicates) 


There may be others that I am not thinking of but these are some of the major players...