A positive pressure produces an attractive gravitational field... Positive pressures are just sort of normal pressures and attractive gravity is normal gravity, so normal pressures produce normal gravity, but it is possible to have negative pressures, and negative pressures produce repulsive gravity, and that's the secret of what makes inflation possible.

Lecture 1: Inflationary Cosmology: Is Our Universe Part of a Multiverse? Part I. - The Early Universe (2012)