The pull of gravity on a planet is the sum of all its particles causing a pull on another object, including those in the northern hemisphere. The direction of the pull of gravity is towards the center of the planet. All the stuff on the planet is concentrated around an abstract point towards which they are all pulled, and because a planet is round, that point is at the center of the globe. North and south are defined by the rotation of the planet, not its gravity. Or the orbit of the planet around its star or the rotation of its star etc. north and south are arbitrary.
Basically, to gravity, it does not matter where on the globe you are, rivers always flow downhill, towards the center of the Earth.