The longfin sawtail catshark reaches a maximum known length of , smaller than
G. antillensis and comparable to
G. arae. This species is slender, with a broad head and a moderately long, pointed snout. The large eyes are horizontally oval, equipped with rudimentary
nictitating membranes (protective third eyelids), and lack prominent ridges underneath. A modest
spiracle is located behind each eye. The nostrils are large and partially covered by anterior triangular flaps of skin. The mouth is wide and curved, with fairly long furrows around the corners. The teeth have a long central cusp flanked on either side by one or two pairs of lateral cusplets. The upper jaw of the type specimen contained 62 tooth rows. The five pairs of
gill slits are small, with the fourth and fifth pairs located over the
pectoral fin bases. The two
dorsal fins are similar in size and shape, with blunt apexes. The first dorsal fin originates over the midpoint of the pelvic fin bases, while the second originates over the midpoint of the
anal fin base. The pectoral fins are large and broad, with rounded corners. The pelvic and anal fins are low and angular in shape. The anal fin base is distinctively long compared to the other members of the
G. arae complex, measuring 13–16% of the total length. The
caudal fin has a small, rounded lower lobe and a ventral notch near the tip of the upper lobe. The body is covered by small, overlapping
dermal denticles, each with a leaf-shaped crown bearing a horizontal ridge and three marginal teeth. A series of enlarged denticles form an obvious saw-toothed crest along the anterior dorsal edge of the caudal fin. This species is brownish above, with a marbled pattern of darker saddles and blotches along the body and tail that become indistinct past the origin of the first dorsal fin. The underside is uniformly light, and the inside of the mouth is dark. ==Biology and ecology==