The technology is the same as that necessary to manipulate space-time, so similar in notion to that used to create wormholes and warp bubbles, though perhaps a less complex application of it.

At present our theoretical understanding implies that this requires either exotic negative energy (which has not been proven to exist), or absolutely insane amounts of energy (think mass of planets level energies). So with our current understanding it's either physically or practically impossible.

However gravity is still probably the least we'll understood of the four fundamental forces, so it may be that future discoveries enable space-time manipulation for more reasonable energy expense. It's unlikely to happen soon, but if it can be done it will be. There is already research being done into reducing the energy demand, and much more into gravity itself.