Answer:
Nick believed that Gatsby modeled himself on an idealized version of who Jay Gatsby really was.
Explanation:
Based on the Plato's teaching within the question it can be said that it seems that Nick believed that Gatsby modeled himself on an idealized version of who Jay Gatsby really was. Such as is his imperfect reflection of the idealized version that Gatsby has created in his mind, similar to what Plato stated to be the case with many reality itself.