Very little is known about Gaspar de Castilleja other than that he served under Hernando de Alarcón as the fleet's accountant. He was also a member of Viceroy Mendoza's personal guard.
AGI, Justicia, 259, pieza 2 [visita a don Antonio de Mendoza] 2 Sep1541 listed as footman];
Alarcón’s Voyage in Flint & Flint, Documents, Document 15 [contador for Alarcón];
?Icaza, Diccionario, #1260 [Pedro de Castilleja, native of Lepe, son of Juan de Castilleja and Marina de Torres, came to New Spain with the royal treasurer, Alonso de Estrada]