Guiomar is named by Salazar y Castro as wife of Juan Martinez de Leiva and then, quoting Alfonso López de Haro (another former genealogist), as daughter of Don Juan Manuel. Alfonso López de Haro gives as primary source the "Becerro de Behetrías", but in its transcripts there is nothing about Guiomar parents ("Becerro de Behetrías" confirm that Juan Martinez de Leiva had a wife named Guiomar and their sons).
In my personal opinion, Guiomar could be an illegitimate daughter of Don Juan Manuel, but there is not any current genealogist who ventures to confirm it.
