Well, what we know about gravity is what Albert Einstein summarized in his theory of generativity.
And according to Einstein, it's possible to get an effect of repulsive gravity if the vacuum itself has some mass density.
That's called, that was termed by Einstein, the cosmological constant.
Anyway, this is just one example of a situation where gravity follows Einstein's theory, except that there is some new ingredient, like negative mass.
Somehow it will be prevented now. One idea that was suggested already by Einstein and Rosen, Nathan Rosen, his postdoc back in the 1930s was maybe you can build a bridge that will connect different regions of space and traverse the distance through a sort of a tunnel, a wormhole.
So at any event, they came up with a solution of a wormhole, but then it was founded such a solution is not stable within Einstein's theory of gravity.