At light speed, there would be no warning of an approaching gamma ray burst. We would see it as it arrives.

There is nothing we could do against a significant gamma ray burst that has the energy/proximity to destroy our solar system. Since it would be travelling at light speed, there would be nothing to measure before it arrives.

What we see when we detect GMB from supernovae are particles arriving long after the event that…

I think yes, in theory. In practice, we would include time as a coordinate. So not only xyz but also when.

More than anything, it’s more evidence for Einstein’s equations. As an analogy - if you drop something in a pool of water, ripples emanate (waveforms). Though not the same, it’s a similar idea where in spacetime, when massive objects (and their relative gravity) merge/collide, it sends waves through the fabric of…

Space is still expanding - at apparently the rate of expansion is accelerating.