Anna was Junoing as if gasps were just as essential as the air she breathed.
"Junoing" in this sentence is a verb used in the progressive/continuous structure "Be [verb]-ing".
A verb form ending in -ing is traditionally classified as a present participle when it occurs in this structure.*
The -ing form of verbs in English developed from several different historical forms.* This conflation means that it is not entirely straightforward to categorize -ing words as participles vs. gerunds. But in one tradition of English grammatical terminology, the term "gerund" is reserved for -ing forms that are used in positions where noun (phrase)s would be used: for example, as the subject of a clause, or as the object of a preposition. "Junoing" is not used in such a position in this sentence, so according to that tradition, it would not be called a "gerund".
A more recent term belonging to a newer analysis of English grammar is "gerund-participle". This term is used by the Cambridge Grammar of the English Language, by Huddleston and Pullum, published in 2002, a source that does not recognize a categorial distinction between present participles and gerunds.
*The historical development of the progressive/continuous construction seems to be unclear: the -ing form could have been influenced from both an older construction involving a present participle and a construction involving a gerund. See this question and its answers for more details on this point: progressive forms: participle or gerund?