There isn't a true "root" word in the sense of a single, smaller word that "gorgeous" is derived from. However, the word "gorge" itself likely comes from the Old French word "gorge" meaning "throat," which is a related concept to the narrowness of a gorge.
It's worth noting that "gorgeous" is not a literal description of a valley, but rather a figurative use of the word, suggesting something grand and impressive, just like a breathtaking view from a gorge.