It depends what kind he is. Needlenose Gar's, which are smaller, will eat Rosy Reds. Everglade and Alligator Gars, which are much larger, will eat Rosy Reds and Goldfish. You can also try feeding Hikari Cichlid Pellets. It may take him awhile to start eating them, (and some never learn,) but make sure he doesn't have any feeders when you first start to feed him pellets, (basically, make sure he is hungry.) You can also try freeze dried Krill or Frozen Krill that are in Flat packs, (as opposed to cubes.)
Gars like subdued light levels and lots of plants that either float at the top or reach all the way up to the top of the tank. Since they come from areas with strong currents, it also helps to have a powerhead (a water pump that produces current in your tank,) to have them swim against.