Sealed lead acid can be gell or an absorbed liquid electrolyte, also called absorbed glass mat or AGM, they have very different charging requirements, and discharge characteristics. As a group they're called VRLA for 'valve regulated lead acid'. They will gas if you go over the specified voltages, and there is no way to replace the lost electrolyte. The gel are better for deep discharge applications, though they lose power faster in very cold temperatures than the AGM..the AGM can be charged at higher rates and voltages..though they both need to have the charge voltages strictly limited..undercharging isn't good for them either, as they can sulphate (and the desulphators don't really work, in Deka's ho.)
The advantage over "flooded" or "wet cell" lead acid batteries is; no fumes (well hardlty any) no maintenance (ie adding water to every cell on a regular basis) no spillage, and about 15-20% more efficient (faster) absorption of charge current..
here's what the techs at Deka battery have to say about AGM (not gell) vrla batteries:
Absorption Stage
Voltage limit to 2.40 vpc
Max time 12 hrs or when current falls below 0.10A per 100Ahr
Float Stage
Battery at 2.25 vpc charge to maintain battery until next discharge.