Keep in mind that when creating a mathematical model, the model does not have to reflect the reality of what is happening, but only needs to be able to make accurate predictions. So saying gravity is a wave is more of a layman term. Using wave theory can predict gravitational effects, regardless if it is a wave or not. If my car runs out of gas EVERY time it rains, then measuring rainfall now becomes a perfectly accurate model for determining if my car runs out of gas, even though it technically has nothing to do with it.