Source, in full
In response to Gershwin's query, "I wonder if my music will be played a hundred years from now"; as quoted in "George the Ingenuous" by Alexander Woolcott, in Cosmopolitan (November 1933); reprinted in Ch. IV: "'...A Young Colossus...'" from Gershwin Remembered (1992) by Edward Jablonski, pp. 44-45
Notes from Wikiquote
If George is around, it will. (This version was recounted by Howard Dietz in Dancing in the Dark (1974), p. 61, in response to a virtually identical query—i.e. as to whether Gershwin's music would still be played in 100 years—posed by Newman Levy.)