How to Make a Gravilaser

with a black hole

Lasers don't need cavities. (This may not be a universally held opinion.)

X-ray lasers, nitrogen lasers, and astrophysical masers are examples I've seen.

https://en.wikipedia.org/wiki/X-ray_laser

https://en.wikipedia.org/wiki/Nitrogen_laser

https://en.wikipedia.org/wiki/Astrophysical_maser

You don't nee mirrors for a laser; in fact some type of media cannot use use mirrors as the light generation quickly changes to light absorption.

https://www.repairfaq.org/sam/lasercn2.htm

"An Unusual Kind of Gas Laser that Puts Out Pulses in the UV". C. L. Stong, Scientific American, June 1974, pp. 122-127

You might want to skim through <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>, 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> (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).

Gwaser. Gwaser is what bwings us togedda today.

It's a long stretch of the imagination to suggest that this will ever have macroscopically observable effects, and relativity doesn't generally admit any kind of "gravity drive" without exotic matter.

That's been covered here before:

https://news.ycombinator.com/item?id=24587923