Guzman received a Ph.D. from the University of Cincinnati in 1968.
Guzman began his career as an instructor at Xavier University in 1965. Two years later he took a position of a professor of history at Bradley University. Since 2008 Gregory has been a professor emeritus at the same university.
Guzman is a member of World History Association, Medieval Academy of America, Illinois Medieval Association and Phi Alpha Theta.
On August 22, 1964 Gregory Guzman married Judith Marie Stroik. They have three children.