I make no claim to being an expert on Plato, but your question was asked almost a week ago and is still unanswered, so I'll do what I can.

One element of Plato's beliefs is that the world we experience with our senses is just a shadow or illusion of a higher, transcendent level of Ideas or Forms.

Gatsby's love for Daisy is a bit like this.  More specifically, Gatsby's view of his relationship with Daisy, from five years before the novel's present, is a bit like this.

Gatsby's view of his relationship with Daisy is an illusion.  She never loved him the way he loves her. 

Yet, he sees the relationship as being on a higher level, a transcendent level.  In other words, their relationship transcends the norm, in Gatsby's view.  Over time, his view of the transcendent nature of the relationship grows, until it takes the form of the illusion the reader sees in the novel.   

Gatsby sees the relationship as transcendent, then, and his view of the relationship that the reader sees in the novel is the product of time--five years.  But his view is an illusion. 

Gatsby, then, in a sense, employs Platonic views by seeing the actual relationship as transcendent.  He sees the actual as more than what it is.  And this occurs over time. 

I know I may have rambled a bit, but the ramblings led to my figuring out exactly what I wanted to say and, I hope, to what you needed for an answer.  Take my answer as an example of "writing to learn," not just "writing to express."