Gas molecules in the atmosphere are pulled toward Earth by gravity.

Here's why:

* Gravity: Gravity is a fundamental force of nature that attracts objects with mass towards each other. The Earth has a large mass, so it exerts a strong gravitational pull on everything around it, including the gas molecules in the atmosphere.

* Atmospheric Pressure: The weight of all the air molecules above a given point creates atmospheric pressure. This pressure is essentially the force of gravity acting on the air molecules.

While other factors influence the behavior of atmospheric gases (like temperature, wind, etc.), gravity is the primary force holding the atmosphere to the Earth.