Gatsby does not believe this. He tells Daisy this because he knows that she places importance on status and reputation. This is why she married Tom and not Gatsby - he was not rich or well-respected enough. Readers know that Gatsby is lying based upon his interactions with Nick. At the party Nick attends, Gatsby hides from his guests for most of the nights. He admits to Nick that he knows few people there and that he has no interest in talking to them. If the people were so "interesting", Gatsby would be more involved in the parties.
What is ironic in this scene is that Gatsby realizes that Daisy prioritizes reputation. However, he has not realized that his lack of reputation and history will prevent Daisy from ever taking his love seriously, no matter how much money he has.
It is hard to say. Although Gatsby spends an extravagant amount of money on his parties, he doesn't even participate in the partying as you see in chapter three. He stays on the outskirts and observes. This detail evokes the idea that he does not want to spend his time talking to the people that come to his parties. However, I think that he wants to believe that they are all famous and interesting, because if he believed anything else, then he wouldn't have achieved what he has set out to achieve. The perfect, interesting, extravagant, American life. Deep down, I think that he knows that the people that come to his parties are rather shallow and uninteresting; but, when it has to do with Daisy, reality for him is skewed and at the time he truly believes what he says to her.