I had a pretty lengthy post in preparation for you asking about keeping them outdoors, but it's irrelevant to your question now.
Here's the post for anyone who may be interested (please correct me if I'm wrong on any of this):
As Solomon said, tropical gars and Cuban gars need to be kept at reasonably warm temperatures in order to thrive and survive. They're only suitable for an year-round outdoor setting if the previously mentioned temperature range can be achieved; otherwise, they should be kept indoors only.
If you wish to keep gars outdoors year-round, starting during Summer, give them a few weeks (preferably a month or longer) to acclimate to living outdoors. Be sure to provide them with plenty of food and check on them as often as possible. I personally had to make the tough choice of either placing my two YOY longnose gars in my goldfish pond or get rid of them before heading back to college for the Fall; they were 12" and 14" at the time. I ended up placing them in my unheated goldfish pond, along with plenty of feeders, during early August in order to give them time to fatten up and get used to living outdoors. To make the transition easier, I made sure that their aquarium temperature matched that of the pond prior to transferring them. I was not able to check them myself during the majority of Fall, but my family made sure that they had food and were alive & well. I finally was able to check on them towards the end of November during Fall Break. Both gars were fat and happy; they were both 16" long. I had to head back to college for Winter Quarter, and I was unable to check on them (on account of a layer of ice on the pond) until yesterday. I found both of them to be alive and well; they were still 16" and had lost little weight. So there's my personal example of keeping gars outdoors; I hope it helps.
Only longnose gars, shortnose gars, and spotted gars are appropriate for living year-round in the unheated outdoor setting in the upper states of the lower 48. Alligator gars and the previous list will be fine year-round outdoors (unheated) in the lower states of the lower 48. Florida gars are only appropriate for the southernmost states of the lower 48 when keeping them unheated outdoors; even then one must be careful. Tropical gars and Cuban gars are strictly for heated environments only. One must also make sure that their pond/other outdoor container does not freeze all the way to the bottom to avoid any fatalities.
So in short, do your research on a gars' natural range, give them time to get used to their new setting and fatten up, and be sure to make sure that the outdoor container is deep enough to prevent it from completely freezing.