Gertrude Stanton was born 18 May 1852 in Fort Des Moines, grew up as a child in the wild west plains of Iowa and Colorado until enrolling in the Moravian College for Women in Bethlehem, PA. After moving with her mother to Brooklyn NY, at the age of 22 in 1873, she married a financially comfortable businessman, Edward Käsebier. It was not to be an agreeable marriage for her so by 1880, they led separate lives, divorce being considered scandalous in those times.
After raising their three children she became a student at the Pratt Institute in 1888 where she studied painting and photography, later studying in France and Germany.
She opened a portrait studio in New York in 1897, switched to photography, saying:
"I earnestly advise women of artistic tastes to train for the unworked field of modern photography. It seems to be especially adapted to them, and the few who have entered it are meeting a gratifying and profitable success."
From Wikepedia, Gertrude Käsebier

Gertrude Käsebier, Evelyn Nesbit, 1903
This photo was featured in Alfred Stieglitz's famous publication, CameraWork
Stieglitz had published Käsebier's photos in July 1899 in an earlier publication of his called
Camera Notes, describing her as “beyond dispute, the leading artistic portrait photographer of the day.”
Gertrude Käsebier, Blessed Art Thou Among Women, 1899
Gertrude Käsebier, Chief, circa 1901
For a period of time Käsebier photographed native Americans, inspired by a visit to Wild West shows popular at the time and recalling her earlier years in Colorado and the American West.
Gertrude Käsebier, Alfred Stieglitz, 1902
Alfred Stieglitz, Hansom Cabs, 1894. Collection Eastman Kodak House
Gertrude Käsebier, Robert Henri, 1907
Robert Henri, Moira, 1924
Gertrude Käsebier, Rose O'Neill, circa 1907
Gertrude Käsebier, George Luks, circa 1910
George Luks, Lily Williams, circa 1909
On a personal note I must tell you that my mentor Harold Irving Smith studied with both Robert Henri and George Luks about whom he had endless stories of both great admiration for Henri and his method of teaching at the Art Student's League of New York, and great frustration with the sometimes very kind but nevertheless relentlessly alcoholic George Luks.