The concept of a Grand Tour began in 1964. Gary Flandro of the Jet Propulsion Laboratory (JPL) noted that a special alignment of Jupiter, Saturn, Uranus, and Neptune would occur in the late 1970s. It would enable a single spacecraft to visit all of the outer planets by using gravity assists.
I remember reading about it in the Time-Life Science Series book 'Man In Space' while it was still a dream . Voyager II took that tour and visited all 4 of the gas giants. An amazing achievement. Voyager II is still partially operational in Interstellar Space to this very day.