Pfhorrest wrote:Could gravitational waves not be slowed by passing through masses interacting with them, much like light itself can be slowed moving through a medium?
A "gravitation beach" would then be some kind of ginormous wall of mass, somehow?
I don't think so, because matter is essentially transparent to gravity. However...
In Newtonian physics, the total gravitational force at a given point in a many-body system is simply the vector sum of the gravitational forces from all of the bodies. You don't reduce the strength of a vector that happens to pass through another body.
In General Relativity, the situation is similar. However, there is an important difference because gravitation in GR is the curvature of spacetime. In the low energy regime, where Newtonian physics is an excellent approximation, the curvatures due to multiple masses combine in a linear fashion, i.e., we can just add the different curvatures together.
Incidentally, the curvature mostly affects the time component, causing gravitational time dilation, the spatial components of the metric are barely affected at all unless the gravitational energy is very high. But when the gravitational energy is
high, then things get tricky. We can no longer use linear approximations when the gravitational energy density of a region is high because that energy itself is a source of gravitation.
In regions of extreme gravity, we get gravitational lensing, due to the spacetime curvature being so high that its spatial effects become significant. This lensing not only affects electromagnetic radiation, it also affects any gravitational waves that happen to be passing through. So yes, we should get amplification caused by the waves getting bunched together, just like light gets concentrated.