Gel batteries are one variety of deep cycle sealed cell lead acid battery (the other being AGM batteries). Both varieties are low-maintenance and suitable for intermittent use - making them a better choice for backup power and off-grid vacation cabins than flooded lead acid batteries - but while similar, they are not identical.
The advantage of gel batteries over AGM for renewable energy storage is a somewhat longer cycle life and lower cost per kWh cycle.
The disadvantage of gel batteries over AGM for renewable energy storage is a more limited ability to charge quickly and deliver peak power. This means you may be able to get away with a smaller battery bank if you choose AGMs.
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Note: the $/kWh cycle amounts listed below are based on the battery's regular price, a 20-hour C-rate, and a 50% depth of discharge.
The gelled electrolyte used in gel batteries is very viscous, and the recombination of the gases produced during charging occurs at a much slower rate than in AGM batteries. If you charge gel batteries at too high a rate, gas pockets will form on the battery plates, pushing the electrolyte away from the plates and decreasing the battery's capacity until the gas finds its way to the top of the battery and is recombined with the electrolyte.
The practical effect here is that gel cell batteries usually have to be charged more slowly than AGM or flooded lead acid batteries. Since in a solar PV system you are limited by the number of sun hours each day and need to store every solar Watt you can before the sun goes down, you may need to oversize the battery bank a bit if you're using gel batteries. This fact makes gel batteries a bit better suited for hydroelectric and wind systems.
