try guppies. at that size your gar needs to eat quite a bit and quite frequently and live food generally won't cut it at this point in its life.
with gars this small i have always fed them live until they are around 6-8" long and then start experimenting with non-live.
if you put in non-live food and the fish doesn't take it (and leave it in the tank) it can quickly pollute the water...and at this small size gars are very susceptible to water quality issues (compared to their larger more robust older forms).
if the fish is extremely small (too small for guppies), then go with brine shrimp...they aren't the healthiest of foods, but they can fatten up the fish faster so it can grow and take on larger prey. if fed properly your fish should be up to 4-5" in just a week or two.
good luck!--
--solomon