Gel batteries, from what I've been reading of late, actually need to be charged to just under 15 volts, so conventional charging systems start costing you money from the moment you start using a gel. Each recharge falls short of capacity, and this reduces lifespan. Anyway. The guys that make the BatteryTender products have telling info on the subject and a full line of products to properly do the job.
Really, 15 volts, which gels are those?
Gel batteries need very specific charge parameters or they can easily be ruined. Treated well they can be some of the longest lasting but they are a pain to manage without the proper charger or regulator.
The "gel" is an addition of silica which makes the electrolyte about the consistency of thick jelly. If you over charge these batteries you can ruin them in short order. You can actually create blisters or voids in the electrolyte that lead to the demise of the battery.
Most gels like to charge at a max of about 14.1 volts where wet cells and AGM's can be charged at higher voltages 14.2-14.8 +/-. It is best to check the charge parameters of your specific battery chemistry with the manufacturer as each manufacturer recommends different charge voltages for their products. Some can take higher charge voltages than others and some lower.
Using a car charger on gels can certianly ruin them. That Leece-Neville alternator regulator is likely factory set to 14.4V so it too would not be a suitable charger for gel batteries, if you want to get your money's worth.
For example here is what the Deka Dominator Gel's need for charge voltage:
From Deka/East Penn:
"For 12-volt batteries, charge to at least 13.8 volts but no more than 14.1 volts at 68°F (20°C)."
Most any "dumb" charger will put out more than 14.1 volts and usually more like 14.4-14.6 volts and some even higher..