The Gel batteries were/are limited in both maximum charge voltage and maximum charge amperage. Amperage is not to exceed a certain percentage of its capacity. I think this was about 20 amps maximum for a 100AH battery( 20 hr rate) and 14.2v was the maximum voltage.
If these charging parameters are exceeded on Gelled electrolyte batteries then voids can form in the gelled electrolyte, and where voids formed, they remain and that portion of the plates becomes forever useless.
The gel batteries main benefit is it can handle the super deep discharges and retain more of their capacity in deep cycle usage. They are also more resistant to Partial state of charge cycling compared to AGM or flooded/wet batteries
AGM batteries absorption voltages range from 14.2v to 14.9v among different manufacturers, and this is for a 77 or 80F battery.
SOME agms are limited to no more than 30% of their capacity, 30 amps for a 100AH battery.
Some other AGMS say no less than X amount when deeply cycled. Odyssey says that when their group31 100AH battery is discharged to 50%, which is around 12.2 rested volts, it Must then receive a 40 amp charge rate until battery terminal voltage achieves 14.7v At 77F, And then 14.7v is to be held for 4 hours, or until amperage tapers to under a certain level, likely 0.5 amps.
That is the Ideal charge algorithm for the Odyssey AGM when deeply cycled. Anything less is simply less than ideal. How unideal it is, is hard to say.
In general the lower state of charge the battery achieves on recharge, and the longer it remains less than 100%, capacity is being compromised.
Can it be restored? Maybe. AGM batteries do not like to be tickled to death when deeply discharged, with low currents. Even the lower $$ AGM batteries that say no more than a 30% rate, will appreciate amperages that approach this rate. Higher $$ AGMs like Concorde's Lifeline, Enersys' Odyssey and Northstar AGM say the maximum charge current is several times the battery capacity, as long as 14.7v is not exceeded, you can initially feed an Odyssey group31 100AH AGM 500 amps. Sure 14.7v will be reached practically instantly at the battery terminals but if the voltage is held at 14.7v, and perhaps lowered as the battery warms up, then the More amps the better!
So please disregard the 'trickle' charge mentality when recharging well depleted AGMS. Aim for 30% of their capacity, and if An Odyssey or Northstar or Lifeline, aim for 40% or even more, and aim to hold 14.2 to 14.4v for Lifeline, and 14.46v for Northstar, and 14.7v for Odyssey , all at 77 to 80F temperature for as long as it takes for amperage to taper to 0.5 amps per 100AH of capacity at the standard 20 hour rate.
if battery temperature is hotter than 77F, lower those maximum allowed voltages. If battery is colder than 77f, then exceed those maximums.
But if that Odyssey battery was put in the vehicle fully charged, and the vehicle daily driven, and never discharged with the engine off, then that ideal recharge algorithm above is not required. Actually starting a modern vehicle consumes Soooo very little of the battery capacity it can be kept 99.95% charged as long as the alternator can keep it over 13.6v with all the loads running.
Less than Ideal absorption voltages on a nearly fully charged AGM battery that is rarely cycles below 95% don't really matter. Less than Ideal recharges on a deeply cycled AGM will compromise capacity on the first cycle and really become noticeable on the 4 deep cycle as the battery will behave like a much much smaller battery
But if the vehicle's Voltage regulator decides that 15.0 volts is to be held on a 110F battery, well the AGM battery will vent, and can dry out. Cumulative damage one might be completely unaware of.
If the AGM is accidently or Intentionally deeply cycled, then the charging currents are much more critical, and achieving a true 100% charge ASAP, ASOftenAP, is the key to excellent battery longevity.
IF a new, healthy AGM is accidentally discharged and the alternator is to be the only charging source, Not only is the alternator going to be worked very hard for at least an hour, but at least 4 more hours of driving would be required before the battery could possibly be anywhere near the ideal 100% charged range.
If the AGM is aged mostly lived its life in the 85 to 90% charged range, then Absorption voltage might have to be held for 10 hours or more before amps taper to less than 0.5% of the 20 hr capacity. The battery might still be a little 'smaller' than it should/perhaps Could be, but discharging to 50% and meeting that high amp ~30% charge rate until absorption voltage is reached, and if the proper absorption voltage is then held for as long as it then takes to the point amps taper to 0.5 per 100AH capacity, it stands a much better chance of becoming that bigger battery again, and will be easier to fully recharge the next time.
The trick to maximizing any lead acid battery longevity is getting it 100% recharged As soon as possible after any level of discharge.
When AGM batteries are not returned to 100% regularly, they will get more petulant than the flooded battery in the same usage. This SuperDuper high$$ AGM battery is a princess, and will not tolerate being recharged to only 85% or living in the 75 to 90% range very well. It will likely last no longer than a wet/flooded starting battery, but for 3x the price.
But if you can give that AGM princess everything she wants, that Mute AGM princess battery will become the hard working thick ankled farm girl kicking your engine to life, quickly, while giving you a deep tissue massage with a happy ending finish, for many happy years.
Don't give it what it wants and it becomes a complaining feminist with a shrill voice and a litany belittlements to foist upon you when you are most vulnerable.
The mentally healthy AGM battery also can have much much less self discharge. 1% to 4% per month at 80f while a flooded battery can lose 15% a month. The princess can fast.
Once below 80% charged that sulfation that forms gets harder and harder and becomes more reluctant to redissolve back into the electrolyte.
Self discharge accelerates as battery capacity declines and it can be a quick downward spiral.
I am very glad I do not have to warranty batteries. If they are treated well they will greatly exceed their warranty period. Abused, and they can be killed in a month.