Gravitational force becomes weaker over distance (inverse square). Also, gravity is the a very weak force. You can not even measure it in small scale (a.k.a sub-atomic scale). It does not help either that a Hydrogen atom does have a tiny, barely measurable gravitational pull, and with those points in mind, a metre becomes a really big distance, and the gravitational forces present in such a space will be (quite) smaller than the pressure exerted by zero point energy. Also, the gravitational pull of a galaxy is not exactly tiny. After all, we are talking about billions of stars, dust clouds, nebulas, planets, plus a supermassive black hole that are thought to exist at the centre of each galaxy. While the gravitational pull does get weaker over distance, too, the remaining force, coupled with similar forces from the other galaxies, which then kind of add up, is enough to still be bigger than the ZPE’s pressure of the space between them.