Juan de Gaztaca was present at the 1540 muster as a horseman. There is only one Pasajero record for a Juan de Gaztaca and he was born in Luyando (Álava) to Juan Ortíz de Carraztacho and Mari Hernández. This is most likely the expeditionary.
AGI, Pasajeros, L.2, E.3585 = Boyd-Bowman, Indice geobiográfico, II:42 [April 27, 1537, Gastaca son of Juan Ortíz de Carraztacho and Mari Hernández, from Luyando: Álava: Pais Basco]
1540,Feb: Muster in Flint & Flint, Documents, Document 12 [horse, elk hide jacket, native arms]