import bge

scene = bge.logic.getCurrentScene()
own = bge.logic.getCurrentController().owner
G = 1
for obj in scene.objects:
    if obj != own:
        vec = obj.worldPosition - own.worldPosition
        F = G*obj.mass*own.mass / vec.length**2  # F = G*m1*m2 / r^2
        own.applyForce(F*vec.normalized()) # apply force in direction