When I had heard about this, I was shocked. I didn't know Mr. Gygax personally, and I have heard differing stories about what he was really like, but the one unshakable fact is that, along with Tolkien, he was the father of modern fantasy.
Without D&D we wouldn't have had any of the D&D-based RPGs now widely available. And we wouldn't have had Final Fantasy or the Elder Scrolls. Every single role-plying game out there today owes a huge debt of thanks to Gary Gygax, because without him, they wouldn't exist.