I'm not much of a musician or composer, and I'm certainly not a music historian - but Henry Sapoznik is all three, and very well respected.

I'm not going to try to get hold of him to verify the story, but Gershwin is famous for his ability to incorporate into his compositions musical elements which he encountered elsewhere. I'm sure no one claims that the opening clarinet glissando came from a musician fooling around while playing the piece, but it seems more than likely that Gershwin heard a clarinetist play a glissando on some occasion, recognized that it had an emotional effect which he could use, and decided to use it in his composition. The clarinetist wasn't necessarily a member of the Paul Whiteman Orchestra, since Gershwin might have had the idea in mind for years, waiting until he wrote a piece in which it would fit.