Why is the fact that neither Jordan nor Nick believe that Gatsby went to Oxford stressed?Before he is shown the medal and photo.
Concerning The Great Gatsby, I'm not sure I would say that the disbelief you refer to is "stressed." It's mentioned, but I don't really think it's stressed. The short answer, though, is that it sets up the possiblility that the stories are true--since Gatsby does have the medal, for instance--and if these seemingly far fetched stories are true, then maybe there is a good deal of truth to Gatsby after all. In other words, one should not be too surprised at Gatsby being extraordinary. The disbelief sets up the evidence that would lead one to believe. The disbelief draws attention to the possibility that he is telling the truth.
I said the above is the short answer, and it is a possible answer. In truth, however, the above isn't the answer I would really want to give. Gatsby's stories, which are revealed through the disbelief you refer to, are part of Gatsby's created persona. Whether or not they are true is almost beside the point. We all create personas and identities that project what we want others to see. Gatsby has done this to a high degree. And, for the most part, he pulls it off. Gatsby is larger than life, as his love for Daisy is larger than life. His stories about attending Oxford, etc. are part of his persona, and that's what makes them important in the novel.
Gatsby is living his life searching for a past that never really existed. His past is an illusion. He is mistaken. But his persona is a created identity that blends great wealth with mystery and love and a bit of kindness, as well as elements of bootlegging and associations with gangsters. His persona is a work of art in itself. His stories about Oxford are a part of that persona.