The Immoralist clearly prefigures work as diverse as DEeath in Venice and The Stranger. However, it's not nearly as good as most of the books that it influenced. The boo0k is about a scholarly man, who after almost dying from a sickness, resolves to live life to the fullest. This apparently means having a lot of sex with a lot of different people, both male and female and adult and child.
The book was written at the same time that Nietzsche was active and it prefigures later existential writing. Having said all that, I'm not shocked by this kind of thing anymore and existentialism is not an attractive theory to me, so the whole thing was a bit ho-hum. It's written well, but the story itself is meant to be shocking. The main character's abandon is really just a sexual abandon.
I suppose this might have been interesting at the time, but it is not nearly so interesting now. I think that Gide might have to many French writers of the 20th century the relationship that Balzac has to French writers of the 19th century. That is the most influential writer, but significantly weaker than those he influenced.