We breathe by expanding our lungs. This literally pulls air into our lungs because we are making more space inside our bodies. As long as the air around us is thick enough we can breathe, regardless of gravity. On Earth, gravity is what holds our atmosphere. When astronauts go into space they can breath, because their space ships and space suits keep the air around them, even though gravity is very weak.

Zero gravity has nothing to do with breathing. If there is air, then you can breathe it; if there isn't air, then you can't.

Gravity on planets tends to attract atmospheres, and this means that planets with gravity may have breathable atmospheres, unlike the vacuum of space. However, if you have some means of keeping an atmosphere without gravity (for example, your atmosphere is contained inside of your spaceship), then there is nothing preventing you from breathing in zero gravity.

Breathing actually has to do with air pressure rather than gravity. When we inhale air, the diaphragm contracts and pulls down the lungs, creating a "vacuum" inside (lower air pressure in the lungs relative to the atmosphere outside). As a result, air from outside your body rushes in and we complete the inhalation.

That is a very interesting question. Let´s clarify how gravity and air are related and what that means for humans being able to breathe. A person needs air (oxygen) to breathe, but as long as they have it, then it does not matter whether or not there is also gravity.

The complicated part is that gravity is the force that holds us to the ground, and in fact, gravity also is the force that holds the atmosphere, our breathable air, against the earth where we can use it. In space, there is no air because there isn´t any gravity to hold in the atmosphere. BUT, let´s say you are inside of a space ship, which is closed container that holds air that can´t escape. That means another force besides gravity is keeping the air in so you can breathe it. This force is the sealed walls of the space ship.

So as long as there is air in the space ship, you can breathe it, even if there is no gravity.