Gars are Piscivores.. that means they eat fish. frozen foods outweigh a majority of live foods in the disease/parasite issue. My Gar get a good variety of Pellets/frozen/ and live foods. shrimp and tilapia are both on that list. I also routinely hunt my local seafood section at the grocery store for deals on smelt, salmon, scallops, and other fish fillets. Mine also go bonkers for frozen krill. On occasion they will also steal a nightcrawler from my fire eels.
I avoid rosys and goldfish unless i know my LPS has clean healthy stock. any live foods should be thoroughly Quarantined before feeding and fed a high quality diet dureing that time. UNlike many other species particularly cichlids Gar do not seem to have issues with Caprids ( which is why feeders in general are so often avoided by most hobbyists) with that being said. I don't think this has ever been proven or disproven. I have personally only seen good growth and health in my 2 Florida Gars, 1 haveing been raised up to about 12" on live alone. as a YOY fish he hit 14" which is on the larger side for a florida.
Gars are unique from most fish in their feeding and general captive habits, They do not follow alot of rules that other species do.