Dunno about Gel much if they were new OLA I'd discharge them one at a time from full full (not a typo) charge at rated C20 curve for 10 hours and see they hold 2.025V per cell after a 24hours rest.
An older set takes longer to get an estimated C20 because I'd take them off discharge periodically and note the resting voltage to avoid over-discharging reduced capacity batteries. Either way 2.025V per cell is my cut-off; time x discharge rate x 2 {temperature compensated} = new rated capacity.
Then leave them disconnected for a week fully full charged and see how much they've self-discharged over that time.
Build the bank from similar capacity that hold their volts or hold the same volts.
Then discharge the bank
to at

C20 for 10 hours to verify bank capacity.