Was Gusto Popular?
The NGram data on gusto is borne out by both the Corpus of Historical American English and the Hansard (British parliamentary) Corpus. For instance, here is the results for gusto in Hansard, divided by decade:
Note the first instance after 1850, the growth in usage around 1920, and its resurgence after 1980. Here is COHA:
So the corpus data, broadly speaking, agrees. This shows that gusto did become more popular in English, at least in the 1920s and 1930s, and we also know from the OED that it became more common in its current usage in the early 19th century. What caused either of these spikes?
The Dictionary Explanation - Italian Loanwords
One explanation would place gusto in the same realm as other loanwords from Italian, French, and Spanish during the early modern period. Gusto was included in many early bilingual and trilingual lexicons prepared for people who may travel to Italy, trade with Italians, or read Italian literature. So, like some of these words, gusto may have come into English in a fairly literal meaning and changed from there.
In the search engine Lexicons of Early Modern English, gusto comes up 50 times between the Medulla Grammatice (Pepys MS 2002) (c. 1480) and Samuel Johnson's Dictionary of the English Language (1750). I've selected five samples that show the word in gradual transition from being a translated Italian word meaning taste to a loanword in English meaning excitement:
Gusto on | anglice a Taster // Gusto as | anglice to Taste (MG, c. 1480)
Gusto, as. to taste, to smacke: also to haue a little knowledge or experience in: to attaine to a meane. (Thomas Thomas, Dictionarium Linguae Latinae et Anglicanae, 1587)
Gust or Gusto, a right Relish, Savour, or true Taste of any thing. A Delicious Gusto, Wines, Fruits, or Meats of a curious or pleasant Taste. A Gust of Wind, a short, sudden, furious Blast, as we say a Dash of Rain, for a sudden, short, impetuous Beat of Rain.(B. E., A New Dictionary of the Terms Ancient and Modern of the Canting Crew, 1699)
GU＇STO, It. 1. A relish, favour, or taste of any thing, the power by which any thing excites sensations in the palate. Pleasant gusto＇s. Derham. 2. Intellectual taste or liking. Let them bring no particular gusts along with them. Dryden. (Joseph Nicol Scott, A New Universal Etymological English Dictionary, 1755)
GU’STO. n. s. [Italian.] 1. The relish of any thing; the power by which any thing excites sensations in the palate. Pleasant gustos gratify the appetite of the luxurious. Derh. 2. Intellectual taste; liking. In reading what I have written, let them bring no particular gusto along with them. Dryden. (Samuel Johnson, A Dictionary of the English Language, 1755)
So eighteenth century dictionaries pick up on gusto as an English word, and the meaning is expanding to include not just taste or even intellectual preference but a kind of pleasure or excitement derived from one of them. So later developments in English gusto would most likely occur in the shadow of English usage, generalizing over time from taste to experience on a similar track as relish and zest.
Was There a Single Moment When Gusto Changed?
Probably not. In most cases it is difficult to trace words to a single influential usage. (Not all words are invented or adapted by a Chaucer, a Shakespeare, or a King James Bible.) Rather, gusto most likely crept into the usage denoted "very common from the beginning of the 19th cent" in the OED:
- Keen relish or enjoyment displayed in speech or action; zest.
For instance, note the gap between these four quotes in the OED:
1693 W. Congreve Old Batchelour i. i. 3 It adds a Gusto to an Amour.
1727 A. Hamilton New Acct. E. Indies I. p. xiv The Taste of those Times relished all he presented with a very good Gusto.
1808 W. Scott Let. 19 Nov. (1932) II. 131 We have been both dining and supping upon them [sc. herrings] with great gusto.
1866 ‘G. Eliot’ Felix Holt II. xxxi. 258 The second Tory joke was performed with much gusto.
So in 58 years the examples jump from gusto relating to food (herrings? OK) to comedic performance. Yet we also have examples of gusto referring to love in William Congreve, and that's over a century before Scott or Eliot. So even if we restricted ourselves to looking at popular writing from the late 18th century to mid 19th century for some single usage that explains why gusto changed, that's a lot of time and a lot of texts to look through.
Even if we restrict ourselves to a single genre of influence, like opera, we will find results in a database like Eighteenth Century Collections Online pointing to operas in print. However, we will also find references to other Italian literature, like the epic romance Orlando Furioso by Ariosto:
L'aver avuto in poesia buon gusto,
La proscrizione iniqua gli perdona.
(But [Augustus's] good taste in poetry compensates for the evil of his proscriptions) (OF Canto 35, stanza 26, found on Google Books in an 1826 edition)
In the search results on ECCO I also see results for Torquato Tasso (another widely-read author, esp. of Gerusalemme Liberata), a life of Lorenzo de' Medici by William Rosco, writings from travelers like Robert Southey (who is writing about Spain and Portugal), and so on. A negative cannot definitively be proven, but the non-uniqueness of source thwarts an easy explanation of why gusto became popular.