Born Gunnerius Kristiansen on 29 October 1832 (not on the date in the passage quoted below), he was baptized on 25 November of that year in Hof, Hedmark, Norway. Gunnerius was the third of the ten children - 7 sons and 3 daughters - of Kristian Henriksen (1807-1902) and his first wife Marte Danielsdatter (1809-1948). His father, who emigrated to the United States in 1870, became known as Christian or Christ (Henrikson) Johnson. Gunnerius' name became Generius Johnson. Generius had come to America earlier. He and his Norwegian-born wife Syverine Kristiansdatter (known as "Christianson" in the U.S.) (1835-1893) had seven children born in Wisconsin by 1870, and more followed. In the 1870 census in Arena, he is listed as a farmer with real estate valued at $570 and personal property worth $595. By 1880, the "Johnson" clan had moved to Jackson Co., WI.

A passage about Generius, with some errors of fact, appears in the "History of Trempealeau County Wisconsin" complied by Franklyn Curtiss-Wedge (1917), pages 607-608; the book is available free online:

"Gunerus C Johnson ... was born at Solar, Norway, October 5, 1833 ... after coming to the United States, (he) participated in the Civil War as a member of the Forty-ninth Wisconsin Volunteer Infantry. He married Syverine Christianson, who was born in Hedemarken, Norway, in 1836, and who died May 12, 1893. His death occurred November 6, 1898. Gunerus C. Johnson came to the United States in 1854, locating in Detroit, Michigan. From there he subsequently removed to Iowa County, Wisconsin, where he was married in 1855. In 1870 he and his family came to Trempealeau County, homesteading land in section 35, Sumner Township, where he and his wife spent the rest of their lives. They had a large family of 12 children."