What was the relationship between Chaucer and John of Gaunt?

John of Gaunt, the first Duke of Lancaster, was a 14th century English prince, one of the sons of King Edward III. Though he never ascended to the throne, he became of the most powerful men in England, as well as one of the richest. He also developed a close friendship with the poet Geoffrey Chaucer and became his patron. At that time, patronage was essential for writers as with all artists. In a hierarchical society in which the vast majority were illiterate, it was necessary for writers such as Chaucer to obtain support from VIPs such as John of Gaunt if they were to be able to find an audience for their works.
But the relationship of John of Gaunt and Chaucer went beyond that of patronage. As well as being close friends, they also became related through marriage. John married his mistress Katherine Swynford de Roet, making her his third wife. Katherine's sister Philippa was already married to Chaucer, so he and John became brothers-in-law. Long before this, Chaucer had written his early poem "The Book of the Duchess" in commemoration of the recently deceased Blanche of Lancaster, John of Gaunt's first wife. He would go on to praise John in a later work, a short poem entitled "Fortune," in which he makes numerous references to his "beste frend."