There is a FAG memorial with picture of the gravestone, which says that Gertrude was in fact the wife of Peter Duvall.
See here https://www.findagrave.com/memorial/105419772#view-photo=145214303
Here she is in the 1860 census "United States Census, 1860", database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:MCQ7-48L : 13 December 2017), Gertrude Duval in entry for Peter Duval, 1860.
In 1870 she lives together with the family of her daughter "United States Census, 1870," database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:M8KD-9XK : 12 April 2016), Gertrude Duval in household of John Shafer, New York, United States; citing p. 49, family 455, NARA microfilm publication M593 (Washington D.C.: National Archives and Records Administration, n.d.); FHL microfilm 552,591.
I didn't find any documents that support her supposed birthname though.