Gravity is an observable physical phenonema. We can say gravity is a fact. Gravity was happening long before the Theory of Universal Gravitation was thunk up by some old boy that didn't live close to a Steelhead river and had nothing better to do with his time than to ponder about things like that.

A theory does not invent a phenomena. It attempts to explain it.

The existence of the phenomena of evolution has been well established by the fossil record. Evolution happens.

How does it happen?

Well, Evolutionary Theory, or The Theory of Evolution, is the body of work which attempts to explain how evolution, the phenomena, works.

If currently accepted theories about gravity or evolution were discredited or supplanted by other theories, that would not mean that the phenomenae the theories were attemping to explain did not exist.