For those similarly confused by the title as I.
> astronomers use the word metals as convenient shorthand for all elements except hydrogen and helium
what, ok
I've looked up why before, tldr it's just because historically astronomers have never had to care in any detail about chemical reactions (this is not strictly true of course, but somewhat close for at least a large subset). So they just need a term for "crap that came from stars".
Gravity waves are waves where the restoring force on some medium comes from gravity. Waves you see on the surface of water, for example, are gravity waves.