The original version had more than one author, and they remain anonymous.
[Part of a medieval manuscript of Amadís de Gaula, now at The Bancroft Library at the University of California at Berkeley and displayed at the Columbia University Libraries Digital Scriptorium.]
In 1803, in the preface to his translation of Amadís de Gaula, Robert Southey wrote:
"Amadis of Gaul was written by Vasco Lobeira, a Portugueze, [sic] who was born at Porto, fought at Alijubarrota where he was knighted upon the field by King Joam of Good Memory, and died at Elvas, 1403."
That's no longer believed today. Over the past two centuries, scholars have collected more information about Amadis, and the evidence points to a14th-century Spanish genesis of the story. Although tales of chivalry originated with King Arthur, brought from Britain to Europe via France, they quickly took hold in the royal courts of Spain.
In Spain, King Alfonso X of Castilla y León (1252-1284) ordered stories of chivalry be read to knights during meals to inspire them, and future kings continued that practice. Chancellor Pero López de Ayala, born in 1332, wrote about hearing Amadis in his youth, and other contemporary references show that Amadis was well known that part of Spain in the early decades of the 1300s. Vasco de Lobeira would have been too young to write Amadis.
In fact, there are enough references in Spanish sources to trace the development of the story: two books in the early 1300s; three books at the end of the century that altered the plot somewhat to incorporate political and social changes in Spain; and four books in Garci Rodríguez de Montalvo's version, which also altered the plot to be more acceptable to his times — that is, he created a happy ending.
Sometimes Amadis has been attributed to the Portuguese troubadour João de Lobeira (1258-1285), but the verse cited to support that seems to have been written in the 1400s. In fact, all the claims that either Vasco or João Lobeira wrote Amadis were made by Portuguese writers the 15th, 16th, and 17th centuries. Another problem with the Portuguese thesis is the lack of any manuscripts of Amadis made in Portugal from that era or any other. Manuscripts were claimed to exist, but they later disappeared.
But four fragments from Book III, Chapter 68, of a manuscript of Amadis were discovered in 1956. They had been used to bind a newer book. (Old parchments were often used for that in the Renaissance, which is why so many medieval manuscripts have been lost.) The handwriting dates them to around 1420, and the language is contemporary 15th-century Castilian. It contains a few Castilian-Leonese archaisms but no hints of any original Portuguese.
So, who wrote the first version of Amadis of Gaul — or the second version, for that matter? We still don't know. But some candidates can be eliminated.