An American In Paris.
Where to even begin. To say that Gershwin would be the gateway to jazz is oversimplifying the point. Because Gershwin and Joplin both opened my eyes to the European tradition meeting the New World. However, an important point: in the 1970s it was all about access. Because I was too young to have collected LPs in the glory days of classic jazz, which my parents had not listened to, my only exposure to non-classical music was through the movies. This was before I started buying my own albums at Disc-O-Mat for $4.49 each.
An American In Paris would have been one such film where I saw the film, which also contains classical and jazz, as well.
And then there was pianist Oscar Levant, who was in a category all by himself…
One important point about Gershwin is that the soundtrack to Manhattan is also one of my favorite film scores ever. It is simply perfection (but I only get ten albums, so I just had to toss a coin re: An American In Paris vs. Manhattan).