I have a question about gravity?

You wouldn't feel anything. You'd be in freefall, just like if you were in orbit. Gravity hasn't exactly been a force since Relativity, but even if you thought of it that way, the gravity from all sides would exactly balance out, so the net force would be zero.

It is not even necessary to remove the core theoretically to get the result: roughly the gravitational force would be 0 (I mean the Earth's force acting on you). Of course you will feel forces acting on you, theoretically a huge number of forces (every single object with mass has a force acting on you et vice versa), but of course not all of them are sensible. Practically, you would feel something with a high possibility, as the structure of Earth is not completely uniform - it might be noticeable, but maybe not. By the way, pressure: you would feel pressure only if there would be connection between the surface and the hole (of course temperature is a different question). As long as you have a stable "building" with no connection to the air mass, you would be able to set the air pressure inside manually (only the structure would feel the outside pressure).

As the uniformly dense hollow sphere called Earth puts an equal amount of mass, the source of gravity, all around you at the same distance from where you sit in the center, all the gravity field, g, forces would cancel out.

That is, for example, the pull from the north pole would cancel out the pull from the south pole, and the pull from the equator at the GMT meridian would cancel out the pull from the equator the direct opposite side of the sphere.

And when you summed up all the g's for all possible angles around you, you'd have SUM(g) = 0 because all the vectors have an equal but opposite vector and all cancel out. In other words, your net weight w = SUM(mg) = m SUM(g) = m0 = 0 is what you'd weight in the center of the hollow Earth. You'd be totally weightless and just float there in the center (until that fly lands on your shoulder and its impact force starts you drifting in the opposite direction).

so if you made a shell earth the same size and mass as the earth, but hollow, you could walk around on the surface just like you can now, but if you made a hole in the shell and jumped in, you could float around weightless inside. You would experience zero g inside and feel no gravity. Air pressure inside should be equal to the surface air pressure in theory.

I don't see why there would have to be a vacuum, air could flood in through any hole in the surface - if pressure inside became equal to pressure outside, then there would be equillibrium between air entering and leaving the hole.

no, inside a hollow sphere there is no gravity, it cancels out. You would be floating in the vacuum.

Even when you get close to the shell, the pull from the other side cancels out the pull from the nearby shell.

Its feel like freefall you not experience g and fly in air

Gravity is a property of mass. You feel the effects of gravity proportionate to to amount of mass.