The first fossil was found in 1985 by Ouyang Hui at Pengtang near Jinquan and was reported upon in 1986 by Gao Ruiqi and colleagues, mistaking it for a specimen of Tuojiangosaurus. The type species, Gigantspinosaurus sichuanensis, was described and named by Ouyang in 1992 in an abstract of a lecture. The generic name is derived from Latin gigas or giganteus, “enormous”, and spina, “spine”, in reference to the gigantic shoulder spines. The specific name refers to Sichuan.
At a glance the name Gigantspinosaurus gives the impression of an oversized Spinosaurus, a particularly exciting but frightening prospect when you consider what that dinosaur was. In realty however Gigantspinosaurus actually a stegosaur, perhaps the farthest a dinosaur can get from being one of the spinosaurs.
The name Gigantspinosaurus is actually a reference to the huge shoulder spines that grown from the shoulder area, similar to some like Kentrosaurus but upon a larger scale. These spines are thought to have provided Gigantspinosaurus with additional defence from large theropod dinosaurs like Sinraptor, but the exaggerated size may have also served as a display function. The other plates and spines were small in comparison to other stegosaurs, and this seems to fit in with the notion that Gigantspinosaurus is one of the most basal stegosaurs known. Primitive forms are noted for having shoulder spines and small plates, while more advanced forms like Stegosaurus have better developed plates but lack shoulder spines.
Gigantspinosaurus was described by Peng and colleagues as a “medium-sized stegosaur”. It was estimated by Gregory S. Paul in 2010 to have been about 4.2 metres (14 ft) long and 700 kilograms (1,500 lb) in weight. Gigantspinosaurus has a distinctive appearance with relatively small dorsal plates and greatly enlarged shoulder spines, spinae parascapulares, twice the length of the shoulder blades on which they rested via large flat bases. The plates on the neck are small and triangular. The head must have been relatively large with thirty teeth in each lower jaw. The hips are very broad and the low neural spines of the four sacral vertebrae and the first tail vertebra have been fused into a single plate. The forelimbs are robust.
The skin impressions were described by Xing Lida and colleagues in 2008. They cover a surface of 414 square centimetres (64.2 sq in) and show rosettes with a central pentagonal or hexagonal scale surrounded by thirteen to fourteen ridged smaller square, pentagonal or hexagonal scales with a diameter of 5.7 to 9.2 millimetres (0.22 to 0.36 in).
A study by Maidment indicated that Gigantspinosaurus is the most basal known member of the Stegosauria. Peng and colleagues, however, placed it in the Huayangosaurinae.