Martin (1992: 406):
Like discourse systems, grammatical metaphor is sensitive to metafunction and it is possible to recognise different types of metaphor according to the kinds of meaning they interface. Examples of ideational (logical and experiential), interpersonal and textual metaphors have been introduced at various points in English Text; an example of each type of metaphor is presented in Table 6.15. The role of each in interfacing discourse semantics and lexicogrammar is taken up in more detail below.
 To be clear, grammatical metaphor involves, in the first instance, an incongruent relation (of realisation) between meaning (semantics) and wording (lexicogrammar) — which is why it's called grammatical metaphor — demonstrating that it was theorised on the basis on a stratified content plane.
 In SFL theory, grammatical metaphor involves either the incongruent realisation of ideational meaning in wording, or the incongruent realisation of interpersonal meaning in wording. It does not involve the incongruent realisation of textual meaning in wording. On the other hand, grammatical metaphor is itself a manifestation of the second-order nature of the textual metafunction. Halliday & Matthiessen (1999: 398-9):
Grammatical metaphor is a ‘second-order’ use of grammatical resources: one grammatical feature or set of features is used as a metaphor for another feature or set of features; and since features are realised by structures, one grammatical structure comes to stand for another.
Halliday & Matthiessen (1999: 401):
But ideational grammatical metaphors typically have a discourse function of this kind; they are as it were pressed into service by the textual metafunction, to provide alternative groupings of quanta of information. To be clear, logical and textual metaphors have not yet been identified 'at various points in English Text'. Up to this point in the text, only interpersonal metaphor (p39, pp50-1) and experiential metaphor (pp327-329) have been discussed, as verified by the index.