Maybe you just made a mistake, but George Gershwin wrote music during the 20th century, not the 19th.

Rhapsody in Blue is his most important piece, in my opinion. I think that the pieces from Porgy and Bess, especially The Man I Love have had the most impact on the world, in terms of wonderful music and also in terms of American history and culture.

The public loved Gershwin and his work during his time. Maybe there was some sort of controversy, due to prejudice, whatever, but I am not aware of it.

In terms of modern applications, I would have to think about it. Maybe Harry Connick Jr.?