You might want to skim through <https://en.wikipedia.org/wiki/Astrophysical_maser https://en.wikipedia.org/wiki/Astrophysical_maser> which discusses among other

You might want to skim through <https://en.wikipedia.org/wiki/Astrophysical_maser https://en.wikipedia.org/wiki/Astrophysical_maser> which discusses among other things single-pass versus oscillator-based lasers, and whether the lack of oscillating cavity makes an astrophysical source something other than a "true" maser. One would want to apply the same thinking to astrophysical lasers and gravilasers. There are very very large coherent sources in the sky <https://en.wikipedia.org/wiki/Megamaser https://en.wikipedia.org/wiki/Megamaser>, and obviously these are not generated with oscillating cavities any more than is a terrestrial laboratory X-ray FEL <https://en.wikipedia.org/wiki/Free-electron_laser#X-ray_FELs https://en.wikipedia.org/wiki/Free-electron_laser#X-ray_FELs> (examples therein). > how does one make ... a GW mirror This is a genuinely super-interesting question! In an anti-de Sitter universe (AdS), gravitational radiation eventually reflects off the "screen" and returns to the source, unlike in our universe which as far as we can tell lacks such a screen (and expands whereas AdS contracts). Among other interesting results, this means that in AdS a generic black hole that digests matter in blobs in its early history in AdS never fully evaporates, because the gravitational waves emitted from the blobby growth (as opposed to spherically symmetric infall) reflect back before final evaporation, followed in due course by early Hawking radiation. In an open universe like ours, probably nothing can outright reflect gravitational radiation (seriously, though, it would be interesting area of research into mechanisms that could generate a "reflected GW" metric). However, if there is a high-amplitude plane-fronted wave with parallel propagation sweeping across the universe, it could conceivably sweep up incident gravitational radiation into the pp-wave and redeposit it onto the sources. Bondi & Pirani did a bunch of work in this area if you're curious about details; one might start with their "sandwich wave". (Nobody knows how to generate a sandwich wave; it would have to simply be a feature of the evolving universe, like the metric expansion of space).