Travelling to the future at different rates is even quite possible, and there are people right now who are doing it. I'm referring pointedly to astronauts, who being significantly further from Earth's gravity well experience time at a different rate. Heck, even people in multi story buildings are travelling through time at different rates.
Okay, how exactly does being further away from Earth's gravity change the rate you experience time?
First of all, gravity has absolutely nothing to do with time. Speed does, but also if you read Theory of Relativity time is relative to the observer.