Gustaf Janzon was the son of the cobbler Jan Jansson. He became a student at Uppsala University in 1833, was ordained for the archdiocese in 1836, and after several clerical appointments in Stockholm, became the vicar of Adolf Fredrik's parish in 1850. In 1857, he became the parish priest in Värmdö pastorat and in 1868 in Gillberga and Långseruds pastorat. From 1863 to 1865, he was a county politician in Stockholm County. Janzon published "Evangelii-postilla or Sermons on the Year's Collective Gospel Texts" (parts 1–3 in 1853, new edition in 2 volumes 1860–1861) and several individual sermons, some of which were printed in the Evangelical Homeland Foundation's sermon collections from 1863 to 1864. Following the example of Wilhelm Martin Leberecht de Wette and Friedrich Lücke, he published "The Gospel of Jesus Christ in a Visual Compilation According to Matthew, Mark, Luke, and Partly John; or Swedish Synopsis" (1845). He was also active as a religious poet, composer, edifying novelist, and translator. He revised a new edition of Anders Carl Rutström's Herrnhut hymns (1851) and published "Contributions to the History of Skevikarna" in 1866.