There is no doubt that both Gargantua and Pantagruel were popular names of giants in the Middle Ages, though, curiously enough, no mention of the former in French literature much before Rabelais's time has been traced.
What does, however, seem probable is that the first book of Pantagruel (the second of the whole work) was composed with a definite view to this chap book and not to the existing first book of Gargantua, which was written afterwards, when Rabelais discovered the popularity of his work and felt that it ought to have some worthier starting-point than the Grandes chroniques.
The earliest known and dated edition of Pantagruel is of 1533 of Gargantua 1535, though this would not be of itself conclusive, especially as we actually possess editions of both which, though undated, seem to be earlier.
But the definite description of Gargantua in the title as "Pere de Pantagruel," the omission of the words "second livre" in the title of the first book of Pantagruel while the second and third are duly entitled "tiers" and "quart," the remarkable fact that one of the most important personages, Friar John, is absent from book ii., the first of Pantagruel, though he appears in book i.
There is also in existence a letter of Calvin, dated 1 533, in which he speaks of Pantagruel, but not of Gargantua, as having been condemned as an obscene book.