[2] The specific name, jansenii, is in honor of Albert Jacques Frédéric Jansen, who was an administrator in the Dutch East Indies.

[3] The preferred natural habitat of G. jansenii, is forest, at altitudes from sea level to 1,000 m (3,300 ft).

[1] A long snake, G. jansenii may attain a total length of 199 cm (6.53 ft), which includes a tail length of 45 cm (1.48 ft).

Adults are olive or yellowish brown on the anterior and middle portions of the body, with some scales black-edged.

They are entirely black on the posterior portion of the body and tail (Boulenger, 1894).