I wouldn't risk it especially since it's above ground since it will have the potential freeze completely solid, so I'd say that this would be a spring/summer pond rather than a year-round pond then. If it were in-ground, then there would be some insulation from the ground and the layer of ice that will form on top of the pond, but it's still a big risk if the gars aren't properly conditioned, of proper size, and/or have the proper genetics.
genetics has nothing to do with it at this level of extreme temps and tolerances for the fish. unless they were gars from antarctica they wouldn't make it. even in-ground the gars at YOY or even yearling size would not likely survive for long at temps below 50F (a lot of this depends on fat stores at that point).
overall message is don't do it--
--solomon