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,...

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."

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...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.