Here's why:

* Newton's Law of Universal Gravitation: This law describes the force of attraction between any two objects with mass. The formula for this force involves mass, distance, and a constant called the gravitational constant.

* Units of Force: The standard unit of force in the International System of Units (SI) is the Newton. A Newton is defined as the force required to give a mass of one kilogram an acceleration of one meter per second squared.

Therefore, the force of gravity, being a force, is measured in Newtons.