What is gravity on a flat earth?
Nobody really knows, same as the globe earthers. As far as I can tell there are no conclusive models, models we could test.
There are a few theories:
One is that it is all caused by buoyancy or density, another is that the earth is accelerating as 9.81 m/s2. Or finally that there is a force all around us, pulling us down.
Density & buoyancy
This wiki says: "According to Flat Earth Theory, gravity is not the main force keeping us on the ground. Instead, there is a force that produces identical effects as observed from the surface of the earth. This force is known as "Universal Acceleration" (abbreviated as UA).
Objects on the earth's surface have weight because all sufficiently massive celestial bodies are accelerating upward at the rate of 9.8 m/s^2. The mass of the earth is thought to shield the objects atop it from the direct force of UA. Alternatively, it is possible that the force of UA can actually pass through objects, but its effect on smaller bodies is negligible (similar to gravity in RET cosmology, which only has a noticeable affect on very large objects)."
The Davis model
The Davis model, suggested by John Davis, states that gravity does indeed exist. In this model, the Earth is an infinite disk with finite gravity.
My thoughts on this
In a usual flat model (the limited flatearth disc) Einstein's gravity cannot exist, because the earth would have pulled itself into a ball, or the direction of the gravity pull would not be downwards, but relative to the centre of the flat plane.
The density theory is a description of what we see: A helium balloon flies away and a stone drops. However it does not explain why the direction is downward/upward. This is usually taken as a given, but that is too easy. Dismissing that question is exactly what should not be done.
The universal acceleration theory does account for a downward force. However is does not explain the differences that can be measured on the different parts of the earth.
Also it would mean that the earth, and all other object that we can see in the sky, are not only flying through space but actually speeding up. This would require an immense power source and imagine the speeds we would be going right now.
The Davis model shows a theory of a downward force, more or less equally spread across, or under, the earth. What this force is, is hard to tell, but Newton and Einstein and all the others did not reach their conclusions after just a few years.
As long as there is no consistent model that can be used and tested and gives an explanation of the how of the forces involved, on this the flatearth model is flawed.
As to the why, what causes gravity, the models have a long way to go.
And a model is only valid until someone proofs the contrary. All examples shown fail to do that with regards to the Einstein model.