G. halys grows to a maximum total length of 59 cm, which was for a female, with an included tail length of 68 mm. The largest male on record measured 53 cm in total length, which included a tail length of 80 mm. The body build is described as moderately stout with a snout that is slightly upturned when viewed from the side. Dorsally, G. halys is grayish, pale brown, reddish, or yellowish, with large dark spots or crossbars, the borders of which are serrated. One or two lateral series of smaller dark spots are present. There is a wide dark stripe behind the eye, bordered by light stripes both above and below. The venter is whitish, speckled with gray or brown. The strongly keeled dorsal scales are arranged in 23 rows at midbody. The anal plate is entire.