Short answers, pulled from the story.
Gary Gygax created the first level of Castle Greyhawk in the late autumn of 1972. He invited his children Ernie and Elise to explore the dungeon he had just drawn on a piece of paper during a family game night.
The original Greyhawk map was a direct overlay on the real-world map of North America with the city of Greyhawk placed near Chicago. The World of Greyhawk was a completely new world called Oerth that Gygax decided to develop from scratch in 1978.
The Circle of Eight included Mordenkainen, Yrag, Bigby, Rigby, Zigby, Felnorith, Vram, and Vin. These eight characters pooled their resources to construct the Obsidian Citadel in the middle of an evil land.
Saint Cuthbert and Pholtus were the first deities to be published in the Greyhawk campaign. A shrine to Saint Cuthbert appeared in Chapter 2 of The Gnome Cache in the second issue of The Dragon.
Robilar freed the demoness Zuggtmoy from her prison at the center of the Temple of Elemental Evil. Gygax punished him by having an army pursue Robilar back to his castle, which he had to abandon.
The Living Greyhawk campaign was active from 2000 to 2008. It was a massively shared campaign that used the World of Greyhawk setting for many adventures published in support of the game.