GR’s wire is a high quality oxygen free solid core copper in a PVC jacket. I’ve used it to wire up a few pair of speakers and it sounds quite good.
For my latest speakers, I elected to use Neotech OCC sold core copper in Teflon. This is quite a bit more expensive, but I wanted I smaller gauge wire for the tweeters since I was wiring sixteen together in each cabinet. And since I was spending a lot more for premium crossover components (copper foil in oil), I decided it was worth spending on the wire. That said, I think the difference between this and GR’s wire is pretty subtle.