[3] He was a friend of Dominicus Lampsonius who included him in his 1572 publication Pictorum aliquot celebrium Germaniae inferioris effigies (literal translation: Effigies of some celebrated painters of Lower Germany), a set of 23 engraved portraits of leading artists from the Low Countries who had died.
[5] In contrast to Patinir, Gassel's landscapes are bustling with life and activity, with depictions of rivers and mountains, cityscapes with palaces and leisure gardens.
Themes he painted included parables from the Bible, classical subjects, courtly leisure pursuits or everyday life.
Gassel's landscapes modernised this style by accentuating the graphic aspect, their preference for dull colors and the reduction of the level of detail.
In the Episodes from the story of David and Bathsheba, steep cliffs can be seen and was a constant motif in several of his paintings; it showed similarities to works by Herri met de Bles, whom he may have influenced and who may have been his pupil in the early 1530s and was at least 10 years his junior.