Revolutionize or Invent RPG
Gygax really did revolutionize the role playing genre, though.
Revolutionize? As memory serves, and mine goes back farther than I care to admit, there were really no RPGs before D&D. The closest I recall was a set of rules for medieval miniatures combat that included fantasy characters like dragons, elves, magic, etc. This was called Chainmail, by TSR.
The first D&D games I'd ever played was using the original set of 3 small books, long before AD&D. I remember saving my money up to buy the supplements that followed like Eldrich Wizardry and Grayhawk, I think it was called.
After D&D came the flood of low-budget, but cool and creative titles like Tunnels and Trolls, Metamorphosis Alpha (a favorite of min and precursor to Gamma World), Bunnies and Burrows (A one-of-a-kind game that I still have packed away somewhere), Swords and Sorcerers, Runequest (the monochrome color cover version), Bushido, Top Secret... Wow, all the memories!
Oh, and the first time I'd seen a d4, d8, d20...
Gary Gygax, you will be remembered.