I don't know about gravity waves, but surfing one sounds kind of cool.

The gravitational constant of Earth is an average. It varies locally with local density. So for example, if you were to fly from the Mississippi River to the Llano Estacado in an aeroplane with a gravimeter you'd notice the gravity around the Mississippi has less than average acceleration. Moving across central TX you'd pick-up a great increase just west of Austin where you cross a massive granite formation, larger than Stone Mountain (but mostly subsurface except for Enchanted Rock) and then a reduction until you got on the other side of the Permian Basin.

There are gravity maps of the USA that are pretty interesting. If I remember correctly, the gravimeters have become refined such that they can track magma movement.

B