I'd go warmer than 60F for a gator. They can take those temps but would like it warmer.
Most of my big gators in the past were in ponds that would fluctuate with the ambient temperature with heaters to prevent it from dropping too cool. I usually set 65 as the minimum temp for the heaters.
Other similar sized gator gars were what had worked best for me. Aside from that I had good luck with niger catfish. Channel cats had worked with midsized gators.
Pims never seemed to do well. Their barbels would always disappear.
At one point I had a couple very large bowfin that got along well with the gators, though they squabbled with each other.
I tried this year putting a gator gar into my spring fed farm pond. I sure hope that you are wrong that they cannot take "near" frozen temperatures.
I am 0 for 22 with attempts with Florida gar. That said I have had from best I can tell 100% success with LNG in the same pond. The temperature in and of itself was not what even did the Florida gar in. The issue is if they go into essentially a hibernation state before the natural predators in the area.
I lost the Florida's (much larger than some of the LNG's that survived N.P.) due to them becoming largely inactive even before the earliest hibernating predator. They all always survived sprin, summar and early fall. The Snapping turtles adored dining on the FLG before winter even fully hit in MD.
I am roughly 200 miles north of the Spat's northern most Cyprus swamp in the extreme southern end of my state.
Keeping my fingers crossed.