First your question presupposes that creating such a shield would be possible. There are good reasons to believe it is not possible (for one, using such a shield would give you an infinite energy source... imagine placing the shield flat on the earth, then a large mass on top of that. Because the mass has no attraction to the Earth anymore, you can raise it up to any height for essentially zero energy cost, right? Well, now turn off the shield .. or remove it... and suddenly you can extract the potential energy out of the elevated mass. This process can be repeatedly indefinitely yielding infinite energy). Ok, so let's assume for a minute that you could shield gravity of a planet, star, etc. from your ship. Your ship still has inertia, which is what creates G forces when you accelerate it. This is the same case as if you were to remove all of the matter in the Universe except your own ship... no gravity to speak of, right (except the very small gravity of the ship itself)... well, you still have an issue of accelerating the mass in order to go anywhere. And that acceleration causes G forces. The same G forces you feel when you stomp on the gas pedal in your car. Good question, though!