Hi,
If it is actually a gator gar, they have been known to grow 6-10 feet long. So no, it absolutely will not be able to live in a 200l pond for any serious amount of time. And even while small, if the pond has no aeration, at such a small size of 50 gallons, fish won't be able to live in it.
Unfortunately, Alligator gars are one of those fish that should not really be sold to anybody but the most die-hard hobbyist with huge tanks.
I would agree that a true ID from that picture is difficult