Arguably everything that Gay includes in his book MODERNISM can be seen as further expressing the spirit of the Romantic Movement. Historically, other expressions of the spirit of the Romantic Movement include the American Revolution of 1776 and the French Revolution of 1789 as well as the Industrial Revolution. We Americans tend to see political liberalism and economic liberalism as somehow inter-related with one another, at least in expressing the same spirit of the age. But we often overlook that the Romantic Movement in American culture and in European culture also expressed the then-contemporary spirit of the age.
In any event, Gay needed to delimit his scope for the purposes of writing his book. Besides that, others have also used the term modernism to encompass the same cultural expressions that Gay encompasses by the term.
The subtitle of Gay's book evokes memories of religious controversies involving so-called heresies. However, the so-called "lure of heresy" that Gay writes about typically involves secular heresies as it were. More neutrally, the modernists were moved and attracted to, or lured to, to depart from the conventional. So his book could be subtitled "The Lure to Be Unconventional." According to Gay, a certain "faultlessly modernist piece" of art could be characterized as "an act of aggression against conventional three-dimensional objects" (page 488).
It appears that the lure to be unconventional, but within socially acceptable limits of course, may be a key aspect of modern Western culture. Modern Western culture promotes relentless change in capitalism, relentless drive for new discoveries in modern science, and relentless changes in fashions and styles in various artistic expressions.
Historically, pre-modern cultures in Western culture and elsewhere around the world did not promote relentless change to such an extent.
As a matter of fact, most Americans today appear to prefer a measure of stability in their lives.
When we turn our attention to the various expressions of modernism that Gay discusses in his book MODERNISM (2008), we may wonder how many Americans were fans of the full range of modernist expressions that he discusses. No doubt certain Americans were fans of the full range of modernisms. Nevertheless, many conventional Americans were probably not fans of any of the expressions of modernism that Gay discusses. Nevertheless, the trickle-down influence of the expressions of modernism that Gay discusses in contemporary American culture has most likely touched the lives of most Americans to an extent.
Yes, to be sure, trickle-down influence sounds like top-down influence, because each of those two expressions contains the word "down."
Avant-gardes typically aspire to exercise top-down influence. Ironically, avant-gardes also typically see themselves and their efforts over against a conventional adversary that the avant-gardes see as functioning in a top-down way. So from the standpoint of the avant-gardes, the avant-garde appears to represent the bottom-up spirit of change.
Gay sees modernism spanning about 120 years, then dying in the 1960s (page 489).
Disclosure: The spirit of modernism was part of my undergraduate education (1962-1966).
In what may seem like an oblique way, Ong himself called attention to what can be interpreted as a harbinger of the approaching end of modernism in the 1960s that Gay notes. In the book THE BARBARIAN WITHIN: AND OTHER FUGITIVE ESSAYS AND STUDIES (1962), Walter J. Ong, S.J. (1912-2003), published the title essay "The Barbarian Within: Outsiders Inside Society Today" (pages 260-285). Gay sees modernists as cultivating the position of being outsiders standing over against their fellow conventional middle-class citizens. Ong focuses on beatniks versus squares. As he points out, beatnik paraphernalia was being mass produced and marketed, presumably to squares who wanted to be hip and with it.
For further discussion of how fashionable it has become for middle-class Americans to cultivate the outsider posture, see Grace Elizabeth Hale's book A NATION OF OUTSIDERS: HOW THE WHITE MIDDLE CLASS FELL IN LOVE WITH REBELLION IN POSTWAR AMERICA (2011).
Ironically, anti-60s conservatives appear to be nostalgic for the years in which modernism flourished. But of course anti-60s conservatives are probably part of what Gay describes as "the untutored majority to whom modernist deviations were a matter of puzzlement or supreme apathy" (page 488). Then again, perhaps anti-60s conservatives are part of the group of philistines who, according to Gay, "remained the preserve of the culturally conservative middle-class audience consisting of prosperous, opinionated burghers who didn't know anything about art but knew what they liked" (page 488).
Concerning anti-60s conservatives, see Philip Jenkins' book DECADE OF NIGHTMARES: THE END OF THE SIXTIES AND THE MAKING OF EIGHTIES AMERICA (2006).