In Chapter Eight of The Great Gatsby by F. Scott Fitzgerald, Gatsby himself declares that he seeks Daisy as the knight in shining armor romantically seeks the holy grail when he relates the story of his youth to Nick Carraway. For, Jay Gatsby clutches to some vague hope that Daisy will turn to him after he spends the night standing in the moonlight "watching over nothing" as Daisy and her husband talk at the kitchen table following the tragic murder of Myrtle Wilson.
"High in the white palace the king's daughter, the golden girl," Nick remarks in Chapter Four, and this phrase is recalled ironically in Gatsby's delusional image of Daisy, the white flower whose center is gold--"her voice is full of money." She is anything but pure and worthy like the grail; hence, the irony. And, this unattainable, this holy grail, finds its worth for Jay Gatsby in Daisy's wealth and name, two false values. That he pursues her as an unattainable makes her more worthy to Gatsby; he denies that she has ever loved Tom Buchanan, and he becomes engrossed in his pursuit of Daisy, much like the knights who lost sight of what was real in their idealized pursuit of the grail--also ironically.
In another ironic aspect of Gatsby's having envisioned Daisy as the holy grail, Nick further narrates in Chapter Eight that
Gatsby was overwhelmingly aware of the youth and mystery that wealth imprisons and preserves, of the freshness of many clothes, and of Daisy, gleaming like silver, safe and proud above the hot struggles of the poor.
Now, however, Daisy ironically is gravely involved in the "hot struggles of the poor" as she has killed Myrtle.