The problem is that they had no child by this name see Wikipedia and Charles Cawley where Pepins parentage is unknown.
Her identity is part of the complex question about the reconstruction of the Arnulfing dynasty. Eckhardt noted that the children of Saint Itta and Pépin of Landen have the Agilolfing names Grimoald, Gertrude and perhaps Gerberge (Karl August Eckhardt, Merowinger Blut - I Die Frauen und ihre Karolinger, Witzenhausen, 1965, p. 23-59). It is likely that one of them was a grandchild of Garibald (died 590), duke of Bavaria and his wife Waldrada, but also unlikely that they were first cousins. Scholars are split on the question, but Pépin had a sister Waldrada so it is more likely that the Agilolfing connection came from his ancestors than from Itta's. If so, the wife of Carloman and mother of Pépin must have been a daughter of Garibald and Waldrada. - Justin Swanström, Dec. 2, 2013.
