maybe its unique to "my eyes" but the initial proposed look via "How the CSS spec says it looks (native CSS rendering)" seems/appears "most representational of the spectrum" ./shrug
I learnt about gamma the other day from this article. Don't have to read it, just scroll down until you see the colour gradients, and maybe the swiggly lines over colours.
The example gradient isn't supposed to be a representation of the spectrum, it's just smooth interpolation between three arbitrary colours.
Have a look at the Oklab article I linked - the human perception of the brightness of the spectrum is very uneven. The point of this tool is to correct that unevenness, when interpolating between colours.
Ultimately, CSS is really just about what looks good, so if you think one style of gradient looks aesthetically better than another, I can't argue with that.
edit: using latest stable FF