Son of the giant Oromedon, and inventor of the Poulan shoes — i.e. shoes with a spur behind, and turned-up toes fastened to the knees. These shoes were forbidden by Charles V. of France in 1365, but the fashion revived again. (Duchat: Ouvres de Rabelais.)
According to the same authority, giants were great inventors: Erix invented legerdemain; Gabbara, drinking healths; Gemmagog, Poulan shoes; Hapmouche, drying and smoking neats' tongues; etc. etc.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Gemmagog from Infoplease: