The 17 year-old had only signed a new deal with Cadiz earlier this year, but he is now joining Real Madrid.
Guille will initially be registered with Real's Juvenil A squad with the aim of breaking into Alvaro Arbeloa's Castilla.
Cadiz announced this morning: "Guille embarks on a new chapter after an exemplary career in our youth academy.
"He arrived at Cádiz at just 12 years old, progressing step by step through each of our teams until making his debut this season with Mirandilla, where his performance and commitment earned him a place on the U-18 national team. His journey, built on hard work, humility, and a deep connection to Cádiz, has not gone unnoticed and now leads him to Real Madrid.
"Cádiz Club de Fútbol can only express its pride and gratitude for everything Guille has given to this club and wish him the very best of luck in his personal and professional future. This will always be his home."