I hate to contridict but a gel-cell, any lead-acid battery, should (ideally) be charged with a current-limited, voltage-regulated charger. A constant curret "trickle charger" is more suited to Ni-Cad batteries.
If the voltage is not limited, the battery will bubble and loose liquid over time. Voltage on a 12 volt group is normally limited to 12.8 to 13.4 volts. The (charge) current is normally limited to 1/10 to 1/3 of the rated capacity.
I have a number of gel-cell batteries that are pushing 30 years old and still truckin :-)
BTW: I am a designer with a major electrical utility. Batteries and d.c. wiring are somewhat familiar to me ;-)