My university's GPA was (IIRC) -2..9 or something where 9=A+, 8=A, 7=A-, etc but that wasn't used in any externally facing mechanism, because a GPA as a single score is not useful outside of the school administration. Single average GPAs aren't remotely robust enough to warrant any kind of external value, and they actively discourage any course experimentation because if you are wanting to do anything GPA gated you cannot risk anything that you don't already know you'll get a high grade in.
Given you could get into uni on a high enough 6th form cert grade this is obviously absurd (I think that it was something like your total score for your best 4 subjects had to be less than 6, so you can see how one course consuming 3 points even if you were absolutely perfect screws that)
I think the correct thing is to require schools to publish their minimum and maximum grades, and then everyone can just scale that range as appropriate - because why should some kid going to Stanford get to report a 4.0 GPA when they’ve got low grades, but a kid at CSUEB has to report something lower because they’re a state school that can’t invent new grades?
From my personal experience, my GPA when I graduated was 8.7 or something, which at least makes it clear that I’m not on the same scale as you might normally think - whereas a 3.9 from Stanford might seem really good but is bolstered by courses with >4 scores. An extreme would be if you heard someone from my school talk about a 3.9GPA, which if IIRC would imply an average grade in the region of 60% (and yes you could in principle go negative, but that would get you on the “let’s talk about whether uni is right for you” list with the uni admin)