Batteries rigged in series, or permanently paralleled should always be identical.
Batteries paralleled only during charging can be different sizes/types IF using a constant voltage charging setup (such as a standard vehicle voltage regulator/alternator) or a 2-stage charger.
Using a 3-stage charger on mixed types/sizes/ages is generally going to result in the smaller or older battery getting overcharged.
(And, as Mike noted...GELs can't be mixed with anything except other GELs.)
Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker