On November 1, 1968, the voluntary MPAA film rating system took effect,[2] with three organizations serving as its monitoring and guiding groups: the MPAA, the National Association of Theatre Owners (NATO), and the International Film Importers & Distributors of America (IFIDA).[15] Only films that premiered in the United States after that date were affected by this.

https://en.wikipedia.org/wiki/Motion_Picture_Association_film_rating_system#References

If a movie wasn't rated it was open to all audiences, hence "G."