that's the part that flies in the face of the current standard physical model: you can't have gravity without mass.

If you somehow come up with some negative mass material (believed possible, but never conclusively observed nor synthesizable), you could add some to your spacecraft until the total mass falls as close to zero as desired, then proceed at arbitrary accelearation, hover without signifigant fuel expenditure over the heaviest stars and planets, etc.

If the negmatter has a sufficiently large negative density, it produces a noticeble negative curveature in space that repels positive mass. This might let you counter the effects of normal gravity over a small area near the negmatter reservoir.

At a certain level, the negative curvature caused by a closed ring or sphere of negmatter can produce a wormhole between the opposite surfaces or openings, giving you your stargate. The ends can then be separated in space and time, as they experience time dilation independently. Energy is conserved, so some maintenance may need to be done to make up energy losses when people step out of gravity wells and so forth.

Robert L. Forward's _TIMEMASTER_ develops all of these ideas and then some.