Even native species. Was he exposed to any non-native species while in a tank?
The issue is not only release of non-native species into our local waterways, but possibly the release of pathogens that even native species could be introduced to in the aquarium, especially if they are housed with non-native, tropical species. We don't want to spread a foreign pathogen into a native species in the wild accidentally, no matter how remote the possibility is.
What kind of gar do you have, and how big? I'm building a pond in my backyard right now. Once its done, I could take just about any size native fish off of you (as long as its not a gator gar....).