From my understanding when trickle charging it's not minimal voltage you want but minimal current. Voltage is like the pressure, current is like the flow volume... you need enough "pressure" to overcome the battery's potential, but you want to limit the "flow volume" so you don't "overfill".
Ie: when charging a car battery that has a potential of 12V you would want to trickle charge with say 13V but only 300 mA. Or something like that.
If you hooked up a charger to keep the battery topped up over along period and had the current too high, say on the charge or boost setting instead of trickle you'd pump way too many amps into the battery, it'd heat up like crazy and it'd pop or leak or overflow or something nasty.
And yeah, gel cell batteries like the Optimas have different charging characteristics than do normal batteries. The real nice chargers, usually for marine and RV applications will have different settings for gel and regular batteries, along with smart charging capabilities that will only provide charge when needed. If you wanted to get fancy you'd buy one of these and install it in your garage then have a cord that reaches your car with a special plug that mates to a special plug on your car that is connected to the battery. That way you could park it and plug it in like it was a block heater, no hood popping or jumper cables or anything needed.
