As Solomon indicated, there are plenty of factors that come into play for transitioning a gar to living outdoors year-round. Size, age, weight, & locality/genetics all come into play when determining if a gar will be able to successfully make it through the winter. Conditioning & the actual pond itself are major factors as well.
I have successfully overwintered my longnose gars for two winters and overwintered my alligator gars for one winter without any noticeable issues. My two longnose gars were collected locally which almost guaranteed that they had the genetic capability to deal with the winter. They seemed to do very well during their first winter despite still being YOY gars (albeit fattened ~18" ones); that pond was only around 500 gallons and almost completely froze over a few times during that winter. They appear to have taken their second winter in stride as well although that was a much milder winter than their first one; they were in my big pond for that winter. I didn't know the history behind my alligator gars, so overwintering them carried a much higher risk even though I conditioned them properly; they appear to be doing just fine based on the lack of fishes of a certain size in my big pond now.
As for conditioning, I kept the gars on live food and provided them with food at all times a few months prior to the onset of winter. For the longnose gars' first winter, I put them in the pond in early August and kept giving them live food until they started to cease feeding due to the drop in temperature. For their second winter & the alligator gars' first winter, I simply allowed nature to take its course given that I had a burgeoning population of bite-size bluegills, smallmouth bass, & minnows in my big pond. The key here is to let them prepare themselves for the winter while you give them the chance to do so (providing tons of live food & letting them transition on their own).