Since you mentioned Huddleston and Pullum, this answer will be based on the terminology that they use. Huddleston and Pullum use the term "gerund-participle" instead of "gerund" because they reject the traditional distinction between gerunds and present participles.
The gerund-participle is a verb form. It is not a noun. A clause headed by a gerund-participle can be used like a noun phrase (NP) in that it can function as the subject of a clause, or the complement of a verb or of a preposition.
Words ending in -ing can belong to various parts of speech: they can be
verbs (in the gerund-participle form)
nouns (deverbal nouns such as "building" or "thinking" in "a building" or "good thinking")
adjectives (deverbal/departicipial adjectives such as "exciting" in "very exciting")
prepositions (e.g. "during" in "I worked for a number of years, during which I met many different kinds of people")
Sometimes words of different categories can be used in the same kind of grammatical context. For example, in the context "It is ____", the blank space could be filled with a verb in the gerund-participle form, or it could be filled with an adjective. The fact that a gerund-participle clause can be used the same contexts as a noun phrase does not mean that a gerund "is a noun". See Araucaria's answer to "How can I prove a word is a noun?" for more detailed discussion of this point.
Of course, there are different approaches to grammar. There may be some definitions of "noun" for which it makes sense to categorize a gerund as a noun, but it's not a noun according to Huddleston and Pullum's definition.