Gershwin wrote a large number of "Great American Songbook" tunes that are what makes him stand out (in my view). While those GAS tunes are not jazz, countless musicians over the years have used them as a basis for jazz improvisation.
We ought not to confuse Gershwin's major contribution as a songwriter with his far less significant Rhapsody In Blue.
And Jim, by no means should we move on past the music of Kern, Gershwin, Arlen, Porter and the many other fine songwriters of that period. It is not over.