He then started to work under the tutelage of Dr. Joseph K. Swift, a graduate of the University of Pennsylvania. Swift realized quickly that Gross' rudimentary education was insufficient training for work as a doctor. Gross then returned to preparatory school, first attending Wilkes-Barre Academy.
Born on a farm near Easton, Pennsylvania, Gross developed an interest in plants, trees, and flowers. He grew up speaking Pennsylvania Dutch, a dialect of German, and supposedly resolved to be a doctor when he was only five years old. At the age of 17 he was apprenticed to a local physician, then another, but both of these experiences soon proved unsatisfactory.