This is a really neat idea, and feels like it has a lot of potential. :) The gravity wells are fun to use, and could maybe be extended to support things like e.g. repulsion wells. Regarding the gameplay itself, I think a few things may improve the experience: 1). Improve the predictability of the bullets. One potential way would be to cap their velocity while in a gravity well, or have them maintain a min distance from the well's center. 2). Make the ships more aggressive. At least in the early levels, I didn't really feel in danger at all - it was mostly just trying to figure out how to send the bullets back. 3). Make the gameplay a bit more fast-paced. The early levels felt a bit slow, and speeding things up a bit may make the game more exciting.
Overall, very nicely done!