The term refers to the huge body of work by American songwriters and composers from about the late 1890's through about 1960. Following is how the term is defined in Wikipedia. I would only add that the advent of commercial radio in the late 1910's and early 1920's led to the growth of popular, i.e., pop, music as we understand it today. Americans invented musical theater, which led to the Hollywood musical. Americans also invented pop music in the post Civil War era when growing middle and upper classes bought sheet music for home entertainment and the musical education of their children.
Great American Songbook is a term referring to the interrelated music of Broadway musical theater, the Hollywood musical, and Tin Pan Alley, in a period that begins roughly in the 1920s and tapers off around 1960 with the coming of rock and roll. Aside from the enduring popularity of this music in its original context, it also became (and remains) the central repertoire of jazz musicians. (In jazz, such tunes are simply referred to as "standards".) For its devotees, the Great American Songbook represents a level of musical and lyrical sophistication that has yet to be equalled.
Thanks. Pretty close to what I dug up. I’d still rather they allowed songs from any musical and leave it at that and not force them to do ALW.