Gilgilgan has multiple forms that progress in stages. Its original and second forms resembles a tortoise with insect-like legs. The first form is colored green with a yellow belly, blue legs, and has a dorsal fin-like spike protruding from its back. Mazinkaiser vs. Shin Getter Robo gives this form four heads.

The second from features a change in coloration with the head being red, the body becoming a light brown, and the legs become a darker brown. This form features a longer neck, tongue, legs, and teeth. A stone-like pale brown humanoid body with spikes on its head and shoulders extends from its back which later becomes Gilgilgan's final form.

Gilgilgan's final form is a pudgy-looking red bipedal demon with wings extending from its back waist, black metallic hands that have open barrels to shoot beams, clawed feet, and three whip-like tails.

The Super Robot Wars video game series gives Gilgilgan a mechanical version of its final form called Mecha Gilgilgan where the monster is covered in gray metal armor, purple claws, and green accessories.