Gravity has nothing to do with the shape of the planet. There's still a reason why things fall down and why airplanes need wings to fly. Why is that?
And we know what's below the surface of the Earth because we can see it with sound waves and how they reflect. The same way water bends and reflects light (put a straw in water and look at it. It looks broken, why is that?) the mantle and core bend sound waves.
Also if the earth is flat, then why can't I climb up a tall building in LA and use a telescope to see China? It should just be right next to us right?
If there's no curvature then why is it night there when it's day here? If the sun is aimed at a disk it would light the whole side of it up at once right?
Yall need to go to College for real.
The reason things fall is because of density. If you are heavier than air you fall. Same reason things sink in the water versus float. A lead ball doesn't sink in water because of gravity. It sinks because its more dense.
The reason we can't see China from a tall building is because there is air, fog, haze, smog, rain, etc. in between us and China that obscures our vision. Its the same way that when there is fog you can't see a few feet in front of you on the highway. There is alot of moisture in our atmosphere that obscures our vision over long distances.
If we live on a flat plane that doesn't move and if instead its the Sun and moon that move around the earth, you would still see the same phenomenon.
The ancients had no problem believing the earth was flat and stationary and it was the Sun and stars that moved. That is what our senses tell us when we look at the world.