The following applies to most sealed lead batteries.
AGM (absorbed glass mat) batteries are designed to hold the electrolyte in the mats so that if the battery is broken the acid does not leak out.
GEL sealed batteries are filled with silica type gels that suspend the electrolyte allowing flow between mats. These batteries will also not leak if the battery is broken.
Both batteries are non-spillable, maintenance free, deep cycle, may be used in any position, have low self-discharge rates, safe for use in low ventilated areas, and can be transported by ground or air.
AGM batteries are more popular than GEL sealed batteries and offer a higher burst of amps. Life expectancy (cycle lifes) remains excellent in AGM as long as batteries are not discharges past 60% between recharges.
GEL batteries are usually more expensive and do not offer the same power capacity as the equally physically sized AGM batteries (example U1 AGM=35AH, U1 GEL=32AH). GELs do better in slow discharge rates and slightly higher ambient operating temperatures. One disadvantage to the GEL battery is the charge profile. GEL cells must be recharged properly or the battery may fail prematurely. The charger must be designed or adjustable for GEL cells since they use lower charging voltages.
If you currently are using a GEL battery it can usually be replaced by an AGM model. You, however, cannot replace an original AGM battery with GEL cells.
If you have questions about which battery is best for your application please contact the manufacturer of your device (instruction manuals are a good reference point) or contact us.