Gert is known to have been a citizen of Aarhus in 1573 when he was commissioned by King Frederik II to build a stone gate and a well at Skanderborg Castle.
In 1574, the king transferred him to Kronborg Castle near Helsingør where he was responsible for coordinating all the sculptural additions.
He also completed two sandstone reliefs (1574) with the coats of arms of the king and queen for the entrance to Helsingør's town hall where they still stand today.
By contrast, the coat of arms, crucifix and tombstone he designed for the chapel at Frederiksborg have now been lost.
The epitaph to Holger Rosenkrantz with two wives in Uth Church near Horsens has also been attributed to Van Groningen.