Does it get sustained low temperatures like that? Or just like a morning and then warms up? They live in many areas naturally that freeze. 3-4c is above freezing so should be fine unless it’s months at a time. Gar will lie dormant on the bottom and not move for weeks when it gets really cold.
I would introduce in early spring so they have more time to get acclimated before winter. Also, 30inches or deeper would greatly help with extreme heat and cold.
I keep my gator gars in an outdoor pond that has the top of the water basically freezing over during winter and they pass through the season laying dormant and fine. One of the hardiest fish of all time.