I am thinking about what would happen in a very-very distant future when most of the stars have already died out into black holes or other catastrophic events, leading to severe lack of light in the sky of remaining star systems with life. And I'm thinking about recent discoveries of gravitational waves made by combining black holes that release energy at the time of combination in range of several percent of combined mass. That energy has to eventually hit something. I wonder if there's a plausible mechanism of focusing a wave emitted from a detected distant black hole pair's combination that could provide enough intensivity in the focal point to make the potential energy create at least electron-positron pairs? At best it would be proton-antiproton pairs, meaning if that's somehow going to happen, there would be a way to make more simple matter for fusion.
A civilization in this question is expected to be a K3 one, operating star-sized power with relative ease, maybe at prolonged intervals, after all real world is VERY slow to accept global changes. So in theory they can move stars or help them move so that the relative positioning required for focusing a gravitational wave is achieved with rough precision, but still produces some output if theoretically possible.
Related: Possible methods to convert gravitational waves into storable energy? yet that question is focused on harvesting energy without any kind of focusing.