I have learned to understand that a nominalisation is called a grammatical metaphor in systemic functional linguistics. What is the motivation behind this terminology? Are there other kinds of grammatical metaphors?
"Grammatical metaphor" is a functional way of explaining a certain kind of linguistic phenomena while "nominalisation" is an non-functional way of explaining a subset of them. I'll try to make it simple with an example.
From a systemic functional perspective, we learn language by noticing how people call phenomena that we can perceive. For instance, if we recall the time we went to school, we can remember that a student usually learns that "a bean tree grows slowly" by watching a bean become a tree. When the school teacher says "your bean tree is growing", students attach that indicative clause to a series of perceptions of a bean tree growing and what they saw becomes the meaning of that indicative clause. The direction of token-value attachment in calling is called "encoding". Afterwards, when students say "my bean tree grew", they are telling others what they saw. The direction of token-value attachment in telling is called "decoding".
During that experiment with bean trees, students may take daily notes of the sizes of their bean trees. If they do this, after the experiment, they can compare their notes on "the growths" of their bean trees. While doing this comparison, they may learn to say clauses such as "the growth of my bean tree was slower than typical". In this case, students are not representing their direct perception of a tree growing. Instead, they are attributing a quality to a thing they can hold, which is the record of their plant's growth. In this sense, the representation "the growth of my bean tree was slower than typical" is "less congruent" to the student's perception of a plant growing than "my plant grew very slowly" if we consider that, for such a statement to occur, students need more note-keeping and a more sophisticated measuring method than mere naked eye observation of a single plant growing. Such a clause can be said to be "metaphorical" in the sense that it is better understood as a verbal representation of other representations (in this case, the notes) of bean trees growing.
In parallel to that, another way to explain why the same "concepts" can be meant by both "verbs" as in "my bean tree grew slowly" and "nouns" as in "the growth of the bean tree is slow" is that there is a derivational process whereby verbs get transformed into nouns by the addition of suffixes such as "th". In this case, the verb stem "grow" is transformed into the noun stem "growth" by the addition of the suffix "th".
Notice: When we explain such phenomena in terms of "grammatical metaphor", we assume that there is a directionality of derivation from more congruent representations to more metaphorical ones. This means that not only nominalisations are covered, but also other phenomena that are not typically understood as being derivations. For instance, if we assume that "my tree grew slowly" is more congruent than "the growth of my tree was slow", we must conclude that the adverbial stem "slowly" is transformed into the adjective stem "slow" by the removal of the suffix "ly" and not the other way round. In that case, if we want to explain this directed transformation in terms of derivations, we would need to describe a "nominalisation" of a verb and a "adjectivisation" of an adverb, which is absolutely atypical in non-functional approaches to linguistic phenomena. In non-functional approaches, researchers tend to describe derivations as transformations from a short stem to a larger one, always by adding suffixes and never by dropping them.