Do you really get the same outcome either way though?

With the fractional gradient, you need a decent gradient right at the bottom. With ISO, you only need a gradient over a 4 1/3 stop range. So I'm thinking about two hypothetical 400 ISO speed films. One with a longer flatter toe might not satisfy the fractional gradient and/or "excellent print" criteria.

So what is the relationship, mathematical or otherwise, between the two methods (eg how did they settle on 1.3 delta log H? in the ISO method)

By the way I'm not necessarily agreeing with 0.3G either. It's too low for me. What I'm ultimately getting at (I think) is that if we're using a gradient in some way to determine speed in either case (which I agree with because I'm all about contrast), I would prefer an ISO method that had a shorter delta log H, or perhaps something higher than 0.1 net D for a speed point.