A man... came and told King Karlamagnus that Duke Milon of Angler had died. The king thought that was a great shame, and asked to whom he should give his sister Gilem. Namlun advised him to give her to Guenelun, because he had lost his wife a short time before. He then married Gilem, and King Karlamagnus gave him the earldom of Korbuillo. They had a son, who was named Baldvini. Guenelun loved Rollant as his son, and Rollant loved Guenelun as a father, and they swore brotherhood.
But later, learned clerks discovered kinship between Guenelun and Gilem... and they were then separated, and Karlamagnus gave her to Duke Efrard, and they had two sons, Athelrad and Efrard. And Guenelun married the sister of Duke Efrard.
This passage explains how Guenelun came into Rollant's life.