As his father was French Canadian, it's likely that the family name was originally something like Gagne or Gagnon. The name of John's wife was probably Sarah Langlois.
It might help if you can find John and Sarah's death records and obituaries. Sarah died in 1942, and land records show that John was still living as late as 1947. Presumably they are buried in Chelsea, or perhaps in a Catholic cemetery nearby.
Thomas was the son of John and Sarah Gurney, born July 4, 1899 in Readfield. (His mother's maiden name is given in various records as "Lagroy" or "Langloi".) The family is shown here in 1900. His parents had moved to Chelsea by 1920. In 1945, Thomas Gurney and wife Mary of Randolph, together with Archie Gurney (unmarried), Victoria Cloutier (widowed), Elsie Wood and her husband Plummer Wood, sold their interests in land on Mount Vernon Street in Augusta that John and Sarah Gurney had bought in 1910. The deed says that "Sarah Gurney died intestate in 1942," and that "we and one Laura Jean are the only heirs at law of our said mother." This will be found in Kennebec County deeds, book 825, page 344.