you can over winter the gar after it hit 12'' and it has to be fat to survive the winter because when temp drop bellow 10C, they will stop eating and will not eat anything until spring come.
I would also like to know, I live in the south-east so probably the warmest area and daytime Temperatures below 15 can be 4-5 months, surely they couldn't winter here on bad years right?
a 100 gallon with good dimension will be a great place to keep the gar during winter time. I've kept 6 spotted gar ranging from 18'' to 26'' in a 110 gallon with the footprint of 3ftX4ft during winter when my pond filter freeze up and flush all the pond water out. if the tank is heated and water temperature is above 15C, they will start eating again in 2 days.