In the context of the passage and The Great Gatsby as a whole, the comparison of Daisy to "a grail" could best be described as:Appopriate, Sarcastic, Foreboding, Ironic, or Condescending?
Of the choices above, ironic may be the best since Daisy is hardly the pristine maid for whom the chivalric knight makes his quest for the holy grail. Perhaps better than the word ironic is satiric as in his novel, F. Scott Fitzgerald satirizes the American Dream in the amoral Jazz Age as an illusion filled with hedonistic conduct, materialism, and spiritual corruption.
Daisy, who is named after a flower that, while it possesses the pureness of white on its petals, contains at its center the corruption of gold. Nick Carraway describes her as having a voice that "sounded like money." She is attracted to Jay Gatsby because he is an illusion for her as the naval officer also in white, and as her wealthy neighbor who shows her his multitudinous colored shirts and drives her in his almost mythological car with its "labyrinth of windshields that mirrored a dozen suns" and fenders that are like wings.
Further, Fitzgerald satirizes the American Dream in the mystical quality of Gatsby's deluded love while his gazing at the green light at the end of Daisy's pier is shattered by the sordid characters who attend Gatsby's parties. And, when Daisy runs over Myrtle Wilson, she is hardly the maiden out of a chivalric tale. For, while Jay Gatsby stands loyally outside her window, she plots with her supercilious husband to implicate the man who loves her purely. Yet, he, too, is tarnished and is not the knight in shining armor as he has connections to the underworld of crime in the person of Meyer Wolfscheim who wears as cuff-links the molars of a man.
That Daisy is referred to as "the grail" is ironic and satiric is evinced in not only Jay Gatsby's soiled character, but also in the marring of "sacredness of the vigil" as Gatsby stands in the moonlight "watching over nothing."
...but now he found that he had committed himself to the following of a grail. He knew that Daisy was extraordinary, but he didn't realize just how extraordinary a "nice" girl could be. She vanished into her rich house, into her rich, full life, leaving Gatsby--nothing. He felt married to her, that was all.
Certainly, this passage is ironic as Nick draws the contrast between Jay Gatsby's interpretation of Daisy as "extraordinary" and the reality of what he means by the word; namely, that her cold disregard and even endangerment of Jay Gatsby is indeed out of the ordinary for a woman who is loved so greatly by this man.