Gulping air is totally normal. They have a modified swim bladder that allows them to beathe atmospheric air.
As for food...that is going to be hit and miss. My 23 incher eats pellets, shrimp, scallops, silversides, smelt, fish fillet, FD Krill, etc...I havent found anything he wont eat yet. Longnose Gar can be one of the more difficult Gar species to get on a prepared diet. I would offer him some raw shrimp and see how that goes for him. If he continues to refuse all foods...you may have to resort to live feeders, just to fatten him up. Then I would recommend getting him on prepared foods (which I could help you with....should you need it).
HA! I see my old catfish in the last picture! How is he doing?