Martin (1992: 490-1):
Grammatical metaphor then is the meta-process behind a text. It co-ordinates the synoptic systems and dynamic processes that give rise to text. It is the technology that let's [sic] the modules harmonise. It is their medium, their catalyst, the groove of their symbiosis, their facilitator, their mediator. It is the re/source of texture.
 This is all disproved by the existence of just one text that doesn't deploy grammatical metaphor.
 This continues the misconstrual of the architecture of SFL theory in terms of "interacting modules". See the earlier critique here.