The best available answer to this question is provided in the 16 page document The New York Genealogical and Biographical Record, Vol. 142 (2011) pages 5 through 20 , that describes the Cowen sisters introduction and involvement with genealogical research.
That article leads to American Ancestors - the legacy of Gertrude Audrey Barber, that article details the many works and points to the NY State Archives for 150 of her collected works.
In summary, G.A. Barber and her sisters were initially associated with the NYG&B and produced abstracts and indexes of material gathered or produced by others, including church registers. Later, they made their own transcriptions, including those of grave markers, as well as continuing to produce abstracts and indexes of will books, church registers, newspapers etc.