Ah, okay. They mean that is further forward than it _used_ to be.
So, the phonetic code /gus/ has moved to /ɡuːs/? Toward what I would call a British pronunciation (inexact, I know). So this pronunciation page offers three possibilities, US Female, US male and Australian female. Is the Australian accent what is meant by “fronting”?
I think so. It often is the same as or overlaps with a change of diphthong, as in the cases in your pronunciation examples.
I have become aware of a difference in (presumably) general American on tv as in the older, fuller (back vowel) long ‘o’ of ‘own’ which seems to be more fronted in, again, what without knowledge I can only call the modern ‘eow’ sound of either general or Californian American (think Valley Girls).
Again, in this case, it concerns two diphthongs in effect. The value of the vowel in the initial phase of the new diphthong ‘eow’ raises the tongue from the bottom to a mid-position vertically in the mouth. The terminal phase vowel is then pushed forward as a result, as a long ‘oh’ sound cannot be reproduced without the tongue being at the bottom of the mouth.
Clear as mud, eh?
Do others agree about that modern ‘eow’ or even know what I am trying so awkwardly to refer to? :-\