Maybe my real question is - what -exactly- is gravity?
Nobody knows, but we will eventually figure it out, discovering the particle or force that carries it.
Then the question will be what makes THAT work.
And we will eventually answer that question.
Then the question will be what makes THAT work.
And so on, and so on. There will always be limits to knowledge, you can always ask WHY enough times to get a "don't know".
But science knows how gravity BEHAVES extremely well. We know how it operates, what it does, how to measure it. We know these things with great precision. We measure gravity waves. We measure the weight of individual atoms. We can predict the motion of planets and send robots out to land on them.
The force that keeps GPS satellites in orbits is well know, after all we put them up there in the first place with that knowledge. If someone wants to claim they are not orbiting a spherical planet, they need to provide an explanation as to just how their alternate theory works, and as you have found out, that's not easy. There is a reason for this.
