In The Great Gatsby, Fitzgerald cautions us against valuing sparkly, attractive things over those of substance. We see this in Gatsby's dazzling, opulent parties—he throws them to keep appearances up, but doesn't especially enjoy them himself. They're bait—a way to lure in the members of the community that will give him social prestige and bring him closer to his lost love, Daisy Buchanan.
When Daisy is likened to a "grail" in the passage above, this is exactly what the narrator is getting at. Gatsby is infatuated with the idea of beautiful, unattainable Daisy, but Daisy herself is almost incidental to his pursuit. He doesn't know her, he covets her. He invests all his time and resources in winning her time and affection. When he does eventually come to know her well, she's a shallow, surface-level character with little to offer. Sparkly and attractive, yes, but without any underlying substance. He's invested time and energy into a world he doesn't especially enjoy, all to win further entrenchment into a world he doesn't especially enjoy.
While Daisy turns out to be exactly—and only—what is advertised be on the surface, Gatsby does to be more. Gatsby cloaks himself in the glitzy costume of the elite to hide his true origins, hoping this will give him access to his sparkly, beautiful holy grail. And for a time, it does—his hard work earns him a spot in West Egg society and the temporary attention of the woman he admires.
Knowing how ordinary Daisy turned out to be under the surface, we can interpret the narrator's use of the term "extraordinary" as irony in this passage. He's poking fun at the romanticized promise of Daisy, in comparison to the reality of her. Gatsby's pursuit is akin to planning an elaborate heist, only to realize you've stolen a fake diamond.