when i first posted this answer, i made it perfectly clear that it was based on speculation for technology that we don't know is feasible yet. I also cautioned that it would be used outside that context as a criticism. So the criticisms you see from other posters here are made outside that context and are a misrepresentation of my initial post.
As for what physicists know about gravity, you can learn as much yourself in fairly short order because they don't really know that much. It is still an unexplained force of nature, and its properties ( how to relate to it mathematically ) are based on easy to understand ideas that are based on the quatities of mass in a given area of space and the distances from that mass.
The biggest differences in the theories are based on newtonian vs general relativity. In general relativity gravity is a result of the spatial geometry, so it's not that hard to imagine that if it were possible for an object to use some kind of device to affect the spatial dimensions around itself, that it could simply create a gravity well nearby for it to fall into, and by constantly projecting that gravity well ahead of itself, it would be drawn in that direction. Because such as system wouldn't actually be moving the air out of its way in a conventional sense, the conditions that create a conventional shock wave may never form.
Similarly, even if we use newtonian physics, and some some kind of "garvitational force" generator were possible, a similar kind of compression and decompression between the air molecules could be made to take place that might negate the kind of compression wave required to form a sonic boom. After all in a gravitational field, atoms are bunched together closer near the source of gravity and spread farther apart in weak field or at a distance.
Anyway ... Sure, take this to a real physicist and ask them to speculate on the validity of these ideas. Also don't forget to use your own mind. And here is a link to a former nasa engineer with some ideas on how a ufo propulsion system might work.
J.r.