 On 29 August 970, he and his brother Thietmar donated part of their inheritance for the foundation of a monastery at Thankmarsfelde.By 975 (probably in 971), this became a royal monastery and was moved (in 975) to Nienburg, a site in the founders' familial lands, where it would serve as a missionary base for work amongst the Polabian Slavs.In 974, Gero established the monastery of Gladbach at the site of a former church, which had been destroyed during the Hungarian incursions.