Conversation so far:
What year was the film, Gigi released?
The film, Gigi released on May 15, 1958.
Who developed the main character for the film, Gigi?
The screenplay of the film, Gigi by Alan Jay Lerner is based on the 1944 novella of the same name by Colette.

Question: how did they pick the cast?
Self-contained rewrite: How did Alan Jay and Arthur Freed pick the cast for Gigi?