Hagen was born in Sigdal, Buskerud, Norway, immigrated to the United States as a young man and settled in Crookston, Minnesota. He sometimes used the first name Gulbrand and eventually used the anglicized first name "Gilbert," but signed his work "G.T. Hagen."
Hagen purchased a Norwegian language newspaper and moved it to Mayville, North Dakota and then to Crookston, Minnesota. He edited and published Vesterheimen from 1894 to 1914. Vesterheimen was a reform-friendly weekly with most of the subscribers in northwestern Minnesota. Hagen ceased writing and editing Vesterheimen in 1914 when English-only movement and the coming war created a chill on German and Scandinavian language use.
In addition to his newspaper career, Hagen wrote several Norwegian language books and short stories. In 1903, Hagen wrote and published the novella Per Kjolseth, a comic sequel to Allan Saetre's popular Norwegian-American novella Marit in Chicago (published originally 1882). In both books, Marit's husband Per was represented as a striver, eager (perhaps too eager) to be Americanized.
Hagen’s daughter Ida Hagen Kirn worked for the famous Norwegian-American novelist Ole Edvart Rolvaag while attending St. Olaf College in the early part of the 20th century and maintained a correspondence with Rolvaag until his death. Hagen died from complications following flu after the 1918 flu pandemic.
- Bruden fra fjeldet. (Mayville, North Dakota: Vesterheimen. 1898)
- Fra Snelandets Hytter. Fortællinger og gamle Sagn fra Norge. (Mayville, North Dakota. Vesterheimen. 1898)
- Per Kjolseth, eller "Manden til Marit. (Crookston, Minnesota: Vesterheimen. 1903)
- Hervin, S. (“Review of "Per Kjolseth," in Smuler. November, 1903)
- Wist, Johan B., (Norsk-Amerikanernes Festskrift, Sigdalsaget, 1914)
- Øverland, Orm. The Western Home: A Literary History of Norwegian America. (Northfield, MN: Author Series Norwegian American Historical Association, 1996)
- Ager, Waldemar. Norsk-amerikanernes festskrift. (translated by H. T. Cleven. 1914. Decorah, Iowa : Symra Co.)