Actually, what he did was transform a previously existing game style into something far more immersive and, IMO, better. There was tabletop wargaming prior to D&D, but it was pushing around soldiers, rolling dice and, in essence, pretending to be General Patton, or Grant, or the Duke of Wellington. Basically, it was Warhammer 40k without the scifi.
Gygax's genius was to reduce the scale, making it more or less one on one, and set it in a sword & sorcery universe. He then had to work really hard to develop rules to balance it.