She served as regent of the County of Katlenburg during the minority of her son Dietrich III of Katlenburg in 1085-?, as regent of the Margrave of Frisia during the minority of her son Count Otto III of Northeim in 1090-?, and as regent of the County of Northeim during the minority of her son Henry II, Margrave of Meissen in 1103-?. She was also one of the leaders of the insurrections against Emperor Henry IV and his son Henry V.
She was married to Count Dietrich II of Katlenburg (d. 1085).[6] In 1090, after the childless death of her older brother Margrave Egbert II of Meissen, last of the male Brunonids, she inherited the ancestral seat of Brunswick in Saxony. When her husband died, she acted as regent for their son Dietrich III.[7]
Frisia
About 1086, Gertrude married again, this time to the Northeim count Henry the Fat (d. 1101), who was appointed Margrave of Frisia on 14 April 1099.[8] Their daughter Richenza of Northeim (d. 1142) married Lothar of Süpplingenburg, Duke of Saxony and future Holy Roman Emperor.[9] He received the Brunonen's seat at Brunswick. After Henry's death in 1101, Gertrud again acted as regent, this time for her second son Count Otto III of Northeim.
Meissen
Gertrud's third husband was the Wettin scion Henry I of Eilenburg (d. 1103), Margrave of Margraviate of Meissen since 1089.[10] Their son, Henry II was probably born after his death in 1103; Gertude acted as regent during his minority.[11] She was one of the leaders of the insurrections against Emperor Henry IV and his son Henry V.[12] She protected the interests of her sons and Margrave Henry II later secured the Wettin authority over Meissen.[13]
B. Elpers, ‘Gertrud von Braunschweig (gest. 1117): die Akkumulation von Macht durch Herkunft und Heirat: eine übermächtige Witwe in Sachsen,’ in B. Elpers, Regieren, Erziehen, Bewahren. Mütterliche Regentschaften im Hochmittelalter (Frankfurt am Main, 2003), pp. 35–57.
Tania Brüsch, Die Brunonen, ihre Grafschaften und die sächsische Geschichte. Herrschaftsbildung und Adelsbewußtsein im 11. Jahrhundert (Matthiesen Verlag 2000). ISBN 978-3786814597
L. Fenske, Adelsopposition und kirchliche Reformbewegung im östlichen Sachsen
L. Partenheimer, Albrecht der Bär. Gründer der Mark Brandenburg und des Fürstentums Anhalt (Cologne, 2001).
O. Posse, Die Wettiner (Leipzig, 1897).
A. Thiele, Erzählende genealogische Stammtafeln zur europäischen Geschichte. Band I, Teilband 1 Deutsche Kaiser-, Königs-, Herzogs- und Grafenhäuser I.