Great, so if you agree that a gas pressure gradient is possible in a container, then you also agree that a gas pressure gradient is possible if the atmosphere is inside a container. But that contradicts what you previously said:
No.
If the atmosphere was inside a container, then air pressure and air density would be constant everywhere as you ascend.
If an
outside force like gravity, as you told us, acts on it, then there would be a gradient. Without an outside force there would be no gradient.
And since gravity is pulling down on the atmosphere creating that gradient, there's actually no need for a container.
And that's what we actually see and define as atmospheric pressure.
I think we can close this chapter and move on.