I always hold up the example of Eric and the Dread Gazebo as what a great gaming story should be: its not about your character, or their stats, or their epic quest to save the princess, but about a generally funny or interesting moment that occurred due to the game mechanics and human fallibility. Those sorts of things are universal, while knowledge about your campaign isn't, which makes most stuff that happens there an inside joke.
Obviously you understand this, but I would basically say, "Hey, let's talk about the game rather your story and character than I have no foreknowledge of", or just generally try to steer the conversation in a different direction. I've found that as long as you aren't painfully blunt, people will understand because pretty much every gamer has been on the receiving end of a long, uninteresting game story.