The surname Gelsenius is the surname of a family of Lutheran ministers centered at Gärdserum, Sweden. It was a common practice for ministers to adopt Latinized surnames taken from the names of their parishes. According to local expert Barbro Behrendtz, the surname Gelsenius was inspired from the parish name Gärdserum. In past the parish name was spelled Gärserum, Gälserum Gelserum, Gälvesrum and Gellersrum.
In later generations descendants of this family adopted the alternative forms Gärdeman and Gerdeman.