For our purposes, creativity is the ability to conceptualize abstract domains of cognition in terms of concrete domains, e.g. domain of time in terms of spatial domains, etc. HCH say grammaticalization begins with individual creativity, taken up communally, then influenced. All these things interact. HCH prefer a narrow definition, but see it as one of the main processes underlying grammaticalization They quote Bybee and Pagliuca, acc. There is a literal meaning and a transferred meaning.
Metaphor involves transfer, here from domain of space to domain of deictic time.
More 'concrete' meaning of go is more easily grasped than more abstract meaning of tense. But HCH feel that these explanations are somehow evasive, or attempting to avoid 'metaphor' when what it really is, is metaphor, or more spec., metaphorical transfer. The strong involvement of a continuum with transfer along the chain of the continuum, is characteristic of metonymy, but metonymy is not the whole picture. Further, only experiential metaphors seem to be involved in such use of grammatical categories. These are entirely different categories and experiential domains, but metaphor is used to include all three.