Take a strong laser to a large lake on a quiet day. Setup the laser on shore and level it. Now, you go out in a boat with a tall white board in it. Every mile or so, you mark the position of the laser dot on the board. As you go further out, you will see the laser dot rise on the board. Why? Because the earth is not flat.

If your theory was correct, you could go forever and the dot would not move. In practice, it does. People have done this or similar experiments many times. Even flat earthers.

There is even a video of a flat earther confirming with this experiment that the earth indeed is curved.

Besides, if gravity is an attractive force between masses then where would the gravitational pull on a disc point? Right. Inward towards the center. So what would it feel like to go from the center of the disc to the edge? It would feel like you are going up a mountain, because the center of mass is not below you anymore, but gets further and further behind you.

And this is the simple reality. Pile enough point masses on top of eachother and the stuff starts shifting and aligning to form a spherical shape. If you take enough mass you get perfect spheres (discounting rotation).

If you look through a telescope you will see Jupiter is pretty much very spherical, because it has such a large mass. Alternatively, look at an asteroid and it will look like a potato. It does not have the mass to deform itself into a sphere.