So, there is always the question, is an effect significant. The simple answer is people dial from a 100 yard zero to make shots shorter than 300 yards, but some math and over simplified physics calculations to illistrate.
Keeping math simple. If we have a bullet launched at 3000 fps, assuming speed is constant, it takes 0.1 seconds to reach a 100 yard target (reality is longer due to velocity loss).
Gravity is acting on the bullet while it is in the chamber even, but the equal and opposite force of the barrel is preventing any drop. The instant the bullet leaves the barrel the opposing force is lost and gravity starts accelerating the bullet downwards at about 32 ft/s^2.
So, in the 0.1 second flight time, a bullet fired perfectly level will drop 1.9 inches, 200 yards will be 7.7 inches, and 300 yards will be 17.4 inches. Remember, these are under estimations because the flight time is constantly getting longer as velocity is lost.
Even with my crappy shooting and a hunting rifle gravity has what I would consider a significant effect on a bullet at 100 yards.