Yes there is the potential for outgassing. If you charge it too hard, it will "blow it's cork" (tiny vent). You don't want that as there is no way to replace the outgassed vapour. If it's a commercial sealed gel cell, it may have charging instructions printed on it. Two different ones as a matter of fact. One will be for occasional charging and will list the volts and amps for that scenario. The other will be for continuous charging and will stipulate a lower voltage and amperage.
I just grabbed one randomly off the shelf here and its a 12 Volt, 7.2 Ah/20HR constant voltage charge.....Cycle use (meaning occasional charging) 14.5V - 14.9V initial current less than 2.88A...............Standby use (thats hooked up to a charger continuously) 13.6V -13.8V (no amperage specified at this lower voltage).
The resistance in a battery increases as it nears full charge. If the battery is big enough, or the charger is small enough, the battery's internal resistance at near full charge will be enough to cancel out the chargers amperage without damaging it(the battery). It all depends on the numbers for your battery!
Stan