Hi Fred,
I like gel cells too but I don't use them in my solar applications here's why:
Gel cells are finicky about charging, they like a two step charge. Usually they charge to 14.7 volts for a 12 volt cell at C/10. (C=amp hour capacity of the battery) Once they reach that charge the charger drops off to the float voltage, about 14.1, and remains there.
Because they have problems with gas escaping through the one way valve and the battery drying out if over charged they can't get the full benefit of a high current charge from the windmill without damage. Also several in parallel means that some batteries won't get the right charge or worse be over charged waiting for the 'slower' batteries to 'catch up'.
You might want to look into sealed AGM batteries from Concorde or MK. They don't have the charge restrictions of gel cells but they do have the high recycle ability that standard lead/acid batteries have.
If you look at commercial applications of gel cells like in UPS systems you'll see most of them use only one battery that sits in standby with a trickle charge on in most of the time. This is an ideal application for gel cells.
RobD