The feeling of gravity or the sensation of acceleration due to gravity is a result of the force that gravity exerts on your body. When you are sitting in a chair, you are not moving relative to the ground because the chair is supporting your weight and counteracting the force of gravity.
However, it's important to understand that acceleration is not dependent on movement in the sense of changing position. Acceleration refers to any change in velocity, which includes changes in speed, direction, or both. In the case of gravity, you experience acceleration even though you may not be changing your position.
Gravity is constantly pulling you downward towards the Earth, and your body resists this force by pushing against the chair. This resistance creates an equal and opposite force, known as the normal force, which prevents you from falling through the chair. The interaction between the force of gravity and the normal force results in an equilibrium where you don't move relative to the ground.
Despite not changing your position, you still experience the acceleration due to gravity because acceleration is defined as the rate of change of velocity. In this case, the velocity being referred to is not the speed at which you are moving, but rather the velocity associated with the force acting on your body. Since the force of gravity is constantly acting upon you, you experience an acceleration even though you remain in the same position relative to the ground.
So, to summarize, the feeling of gravity or acceleration due to gravity is a result of the force acting on your body, even if you are not physically changing your position.