I understand that gravity, as far as we know, is always attractive. Also, it has additive qualities - i.e. the size and strength of the field are proportional to the quantities of mass.
This seems to counteract the idea that gravity can cancel itself out. The centre of the Earth is said to be a zero-g environment, yet it is in the midst of a whole load of mass. I guess this makes sense when thinking about the mass as pulling equally from all directions... Which leads on to two questions.
If opposing masses can effectively cancel each other out, does this mean Gravity is not always additive?
Is spacetime geometrically indistinguishable in an area of zero-g, lets say, between galaxies, and in the centre of very massive bodies, like a planet? What I mean here is, can you tell that there are strong gravitational forces pulling you in all directions as opposed to weak ones?