Is gravity really an attractive force?

Generally, yes. But perhaps there are situations in which it can also behave repulsively.

The gravitational force is governed by Einstein’s field equation, which relates the curvature of spacetime, expressed via the Einstein tensor, to the energy-impulse tensor, which expresses the density of energy and momentum.

Technically, with negative masses and energies, the force of gravity generated would indeed behave exclusively in a repulsive manner.

There is no knowledge of negative masses, and even antimatter seems to be attracted to gravity rather than repelled by it.

There is perhaps a way to obtain negative energy instead. Through the Casimir effect.

When two very small plates are placed at a very small distance from each other, the number of frequencies of the quanta that can exist between them is narrowed down.

Consequently, the number of virtual particle pairs that can be created and annihilated immediately after is also smaller between the two plates than on the outside.

The result is a force, called the Casimir force, which brings the plates closer together. There is then an inverse pressure (tension) between the plates, which pulls them towards each other instead of away from each other, which is equivalent to a negative energy density.

It might be physically possible then, that with the Casimir effect, a different kind of gravity can be generated, which instead of attracting things to each other, pulls them away.