Gag are most identifiable by their lack of distinguishing features. Their body color is an overall mottled gray. The darker body marking may be arranged in "kiss-shaped" patterns, slightly resembling the box-shaped patterns on the true black grouper.

They lack the streamer-points on the tail fin that scamp and yellowmouth grouper have and lack the yellow color in and near their mouths. They most closely resemble the black grouper, but are lighter in color, especially on the fins.

Gag are a fairly well researched grouper. They spawn from December to May, with peaks on the full moons between February and early April. After hatching, the tiny baby grouper are carried into nearshore and inshore waters by currents. Many young gag spend their first summer on oyster reefs feeding on small shrimp and other creatures. After 4 months, the survivors have grown to 5 inches and switched to a fish diet, which is their preferred food for the rest of their lives.

With the cooler temperatures of fall, the small gag move to deeper channels for migration offshore. By October, they are 12 inches long and all are females. After moving offshore, gag grouper stake out a territory that they seldom leave until old enough to spawn. Gag grouper grow rapidly until they are 10 years old and an average size of over 40 inches. After that, growth rates are very slow. Either alone or in small groups, gag will travel to specific areas to form spawning groups. After spawning, the largest fish will turn into males. Gag under 32 inches and 5 years old are all females. By 42 inches and 11 years of age, the sex ratio is about even. Larger fish are mostly males. Female fish outnumber males.

In recent years, this imbalance in sex ratios has become more pronounced, causing some fisheries managers to become concerned. The larger male fish are aggressive feeders and once located, they are easy to catch. The increased efficiency of fishermen is allowing them to target specific gag habitat.