Dettman was born as Gretchen Mahala Limbach in Garnavillo, Iowa on 14 January 1890 as the second of five children born to Henry and Amelia (née Tangeman) Limbach. Her elder sister Viola died in 1888 aged three, making her the eldest surviving child from her birth. All four of her grandparents were German immigrants.

On 9 November 1916, Dettman married her husband, Elmer Henry Dettman, in Garnavillo. Their son, Harold Charles Henry Dettman was born in 1918. Her parents died in 1932 and 1940 aged 73 and 82 respectively. She was widowed on 16 December 1951. Her sisters lived to be octogenarians.

Dettman’s sister-in-law was validated supercentenarian Amelia Schmidt (1875 – 1986) who was her husband’s sister. Together, they have a combined age of over 222 years, one of the highest-known for siblings-in-law.

Dettman became Iowa’s second-oldest living person following the 11 May 2000 death of Jessie Kettering. Only Nellie Cross (1889 – 2002) was older. She died in Guttenberg, Iowa on 30 September 2001 at the age of 111 years, 259 days.