Just google around and find an explanation of how gel-cels works. Johnson Controls probably has one. There is a wiki below. These gel cells are a modern variation of the lead- liquid acid battery that goes back to the telegraph era of the 1800's. And is used in your car.
It is possible for the gel-cell to create hydrogen gas just like their liquid counterparts. go here and search for 'gas'
http://en.wikipedia.org/wiki/Battery_(electricity)