Gertrude the Great (1256–1302) was a German Benedictine, mystic, and theologian. Parishes are dedicated to Gertrude in Missouri, Ohio, Texas, and Chicago, Illinois.
The Tarn (Latin: Tarnis, possibly meaning 'rapid' or 'walled in') is a 237 mile long river in southern France.
As to the name of the school, it's common for Catholic colleges to have both a saint and a location in their name. This naming convention includes Bellairs's alma mater, whose official name is Notre Dame du Lac ('Our Lady of the Lake').