It seems like there is a lack of information on the internet about charging Gel batteries.
I have not been able to find much info at all.
The battery manufacturer web page says to not charge the batteries at a voltage higher than 14.1 volts and to use a "temperature adjusted" charger - but it doesn't recommend a brand or where you can get such a device!
Trying to find a charger that can be set to a lower charging voltage, I did stumble across
THIS ONE
It can be set with a dip switch to limit the voltage to 14V max for charging. It also has three output connections on it, so it could run 10 amps out to each of three batteries. I would only need two of these chargers to charge 6 batts for a 72V system.
Too bad the charger is $250. That's almost half a Forkenswift!
Another thing I would like to look into is if there is some simple way to voltage limit a cheaper charger. For example, if I could simply limit the max out voltage of a 10 amp $30 charger, I could buy a bunch of those and use one each on all the batteries.