Gale began this work in 1896 when he returned to the United States after studying with Wilhelm Wundt at the University of Leipzig. Wundt was the founder of the field of experimental psychology.
Here is a photo of Gale in his University of Minnesota office courtesy of the University of Minnesota Archives.
It looks like he enjoyed reading. I have two of his books. One is from a course he took in Leipzig (unfortunately I cannot read German). The other is a psychology text he must have used when he taught here at the University of Minnesota.
You can read more about Gale's historic experiments in this article in the Journal of Advertising. John Eighmey and Sela Sar (2007) “Harlow Gale and the Origins of the Psychology of Advertising,” Journal of Advertising, Volume 36, Number 4, pages 147-158.