What's the difference between negligence -- which you can waive in advance -- and gross negligence, which you can't?
The best definition we can come up with is that gross negligence requires an "extreme" departure from the reasonable standard of care. Which doesn't really help much. It's a total "I know it when I see it" standard. Sure, we give it a label: "extreme" negligence. It's "extreme". But we might as well just say "gross". Means the same thing. Whatever we say.
It gets even more complicated, as here, in the primary assumption of risk setting. Where ordinarily you can't sue anyway, and we put an added layer of difficulty on top by adding the advance release.
Not that I think there's anything wrong with the Court of Appeal's holding. Or even inherently vague common law standards. But I do think it's worth noting that there are a nontrivial number of areas in the law where our best definition of a concept is one that's essentially meaningless. The obscenity definition advanced by Justice Stewart gets insulted all the time. But we nonetheless use essentially that same principle about as often -- all the time.