Gatsby does indeed wear a pink suit, and it assumes significance in the novel when taken in context. Tom cannot believe that Gatsby ever attended such an old and prestigious university as Oxford because Gatsby clearly does not belong to the upper class.
Gatsby's pink suit makes a statement to Tom; it says that Gatsby flaunts his wealth--which is "new" money. He does not dress in the tasteful, conservative, understated style adopted by those who have inherited their family fortunes, money that often has been handed down through several generations. One of the unwritten rules of Tom Buchanan's social class is that those with a great deal of money do not display it in a vulgar manner. To do so would be quite gauche. Gatsby in his pink suit shows clearly that he does not know the rules.
Everything Gatsby owns, from his pink suit to his colossal, faux-European mansion and "circus wagon" touring car, show his lack of upper-class taste and refinement. No matter how much money he accumulates, Gatsby will never be the kind of man who "belongs" at Oxford.