Can you give a G rating to a film about a girl being trained to be a high-class prostitute for a man twice her age who's known her since she was a child?
Yes. Here's why.
The producers and screenwriters, who knew exactly what Colette's story was about, did their best to make sure that viewers wouldn't. They played down the courtesan angle and turned it into a Cinderella story where the lavish sets and a romantic, love-conquers-all ending take center stage. Memos flew back and forth between the producers, writers, and directors about the production code and what it might mean for making the film (source). As a result, the film turned out to be pretty tame, and any kids who would watch Gigi would probably be oblivious to any sexual themes.