"He wanted nothing less of Daisy than that she should go to Tom and say: "I never loved you." After she had obliterated four years with that sentence they could decide upon the more practical measures to be taken. One of them was that, after she was free, they were to go back to Louisville and be married from her house-just as if it were five years ago" (page 109).

This quotation from The Great Gatsby is filled with specific diction and I believe it reveals Fitzgerald's opinion of Gatsby.
Saying "he wanted nothing less of Daisy" tells critical readers that Gatsby wanted a lot from Daisy. If he had reasonable expectations, I think Fitzgerald would have said he wanted "nothing more of Daisy." This would make Gatsby's request seem minimal and easy. However, Gatsby's request was great and inconsiderate.
Fitzgerald reminds his readers also that fulfilling Gatsby's request would obliterate the last four years of Daisy's life. The strong negative connotation of "obliterate" is of great importance. Obliterate means to kill or delete. It's totally unreasonable for one to request someone else to obliterate the last four years of his or her life. It's also impossible for Daisy to obliterate four years, because she has a young child with Tom. Therefore, Gatsby isn't only unreasonable, but unrealistic and impossible.
The irony in the second sentence is very striking. After Daisy obliterates the last four years of her life (which is physically impossible) Gatsby plans to "decide upon the more practical measures to be taken." This is an oxymoron! It's almost as bad as the books Fun With Shakespeare. Describing their next step as a couple as "more practical," shows readers that the first step (deleting four years) is totally "impractical."
The third sentence takes the irony to another level of ridiculousness. Gatsby and Daisy's "more practical" measure is to pretend "as if it were five years ago." This is a less practical step if anything. How could Gatsby honestly believe that it's more practical to pretend that they're living five years into the past?
Overall, I think Fitzgerald sees Gatsby as foolish. His diction and use of irony shows readers his personal opinion of Gatsby and Daisy's relationship. While readers are free to have their personal opinions, Fitzgerald does a good job of making his opinion known.