battery has come to a full charge and the charger hasn't gone into float mode, backing down to about 13.2-13.6V. Once fully charged to high a charging voltage begins to just convert water into H and O. Now as the batteries get near full charge or are equalizing you will also get some outgassing. But it should be minimal. If you take the cover off the cells, if you see anything more than small bubble(s) every few seconds, something is amis.
If the battery was really "boiling" rapid bubbles definitely an overcharging situation from the charger.
Normally when they do outgas you shouldn't get a noticeable smell, although if it gets to hot then you could get the smell of sufuric acid, (rotten egg smell)
