Source: Zion Lutheran Cemetery, Pickrell, NE cemetery records, courtesy of Pastor Paul Petersen
The Zion Lutheran Cemetery records reflect an 1877 birth date for Gretje, but I believe this is the year she married Frerich Schuster rather than the year she was born. She is in the same plot as her husband Frerich and Frerich's second wife is close by in plot 1-33.
Frerich's obituary in the March 10, 1907, Ostfriesische Nachrichten, states he married Gretje Agena in April 1877 and she died 23 Feb 1881.
The Ostfriesen Genealogical Society of America Emigration Database has a Gretje Harms Agena who was born 21 Aug 1854 in Neudorf and emigrated in 1855 to Golden Illinois. Her parents are Harm Gehden Agena and Charlotte Catharina Berends Alting. This Agena family is found in Sterling, Johnson County, Nebraska, and Gretje fits nicely with the known children. My working theory is this Gretje and the Gretje in the database are one and the same.
Frerich Mennen Frieden Schuster
1852–1907 (m. 1877)