There are two ways to go about it:
1) Charge each one *individually* before placing them into series / parallel service. Since they are gel's, take it easy with the CV voltage, typically no more than 14.1v. OR,
2) Or, place each in parallel, and charge at a voltage no higher than 13.6v. This is a typical "float only" charge, and allows time for unbalanced cells to come together slowly without venting. But you'd need either a programmable supply, or a charger that actually has a "float only" charge setting, which is rare.
Because you are asking in the first place, my suggestion would be to invest in a decent quality normal charger, and charge each one individually before placing into service - again taking it easy on the gel's at no more than 14.1v cv in your charger setting. You may actually see this voltage rating on the side of the battery case or have a gel setting. My preference is to avoid Schumacher speed-chargers since they are too aggressive - their main intent is to get dead automotive batteries back to life, and not normal maintenance such as this.
Just slapping them together in parallel will not achieve self-balance, and when nearing full charge, one or more of the 24 cells in the entire bank could easily overcharge (vent) until that initial balance is achieved.