Since gravitational waves are a type of propagation of energy of some sort, they ought to induce their own gravitational field. I'm assuming this extra gravitational force / curvature is independent from the wave itself, so there ought to be an observer that would 'feel' or 'observe' the wave pass, and in addition 'feel' or 'observe' a secondary attractive force towards the 'densest' part of the wave. I'm not sure if it is possible to discern these two effects without experiencing them as a whole but I'm assuming one could somehow.
Regarding the secondary attractive force, could this be used / amassed to a sufficient degree to allow two (or more) gravitational waves to orbit each other, with the extremal case being a gravitational standing wave arranged in a loop (held together by its own mass-energy)? Is such a system capable of collapsing into a black hole given enough energy in the wave(s) and sufficiently small orbit?