I am a gravity atheist, you should understand that.
Like the atheist sates prove to me there is a god.
So prove there is a force needed for a flat and stationary earth, that has been called gravity.
In order to move objects, there has to be something to move them. Even theists like this one; it's one of Aquinas' famed arguments.
You can test different kinds of objects, use different ways to move them, try to get them to different speeds in different amounts of time... if you do your tests and measurements correctly, and with some simple limitations, you will at some point arrive at what is today called Newton's Second Law of Motion:
The acceleration an object will receive is proportional to the force applied, and reciprocally proportional to the massiveness of the object.
Mathematically: A = F/M
We can observe objects moving on our Earth - flat and stationary or round and moving. Things "fall down". Yes, not all do... but we'll get to that later.
Still, most of the usual things in our daily life do "fall down". They move in specific direction, even if not pushed or pulled by us.
So, according to Newton's Laws that we have previously established, we can conclude that there is some sort of "force" involved making them move. A "mover".
A "mover" that is very precise, rather constant and independent from us as observers.
Now we can make further experiments regarding this "mover". What does it react with? What conditions influence it?
Applying these questions, using different testable ideas like the Archimedian Principle, and creating experimental situations that check some assumptions against each other, we will come to the conclusion that this "mover" is almost exclusively dependent on the massiveness of the object we test.
Not on any other physical concept. Not on magnetism... we can experimentally refute this. Not on "relative density"... we can experimentally refute this.
And there you have it, in it's most simple form: a reliable, caculable "mover".
We call these "movers" "force". That's convention. You could call it "splunk"... it would mean the same concept. Something that gives objects speed.
We call the attribute it works on "mass". A convention. Something that makes objects more or less easy to give speed to.
And we call that effect of objects being given speed in the direction of the ground "weight". It describes how "heavy" an object is. Based on this "heaviness", and that the scientists of old liked to use Latin to make their statements, we call this whole concept "gravity".
That's only the start... but still it is the gist of why "gravity" is a real thing. Whatever it "really" is.
Any further questions? Or evasions?