Question

why is gravity an example of a scientific law?

Answer 1

Because a sxientific law is always applies under the same conditions, and implies that there is a causal relationship involving its elements. And so that is why gravity always applies under the same conditions, and implies that there is a causal relationship involving its elements. 

Answer 2

It's because it's the force that attracts a body toward the center of earth, or toward any other physical body having mass. For most purposes Newton's laws of gravity apply, with minor modifications to take the general theory of relativity into account