Dr. Gillespie's professional career has been primarily concerned with advanced engineering and research in the automotive and highway areas. From the beginning, his career spanned the breadth of these areas, ranging from applied research at the Pennsylvania State University in pavement friction test methods, to responsibilities as a Project Officer with the U.S. Army Corps of Engineers directing engineering and service tests on new military construction equipment. At Ford, he served as a group leader in development testing of new heavy truck products, as well as development of analytical methods and computer programs for predicting truck braking, handling, and ride performance. His expertise in the area of road roughness and vehicle dynamic interactions led to consultation with the World Bank directing the international experiment that developed the worldwide standard used for measurement of road roughness.

In 1987-88, Dr. Gillespie served on the White House staff as a Senior Policy Analyst for Dr. Willliam R. Graham, Science Advisor to President Reagan. He subsequently served as a conslutant to Dr. Allan Bromley, Science Advisor to President Bush, chairing the Interagency Task Force to develop a National Action Plan on Advanced Superconductivity Research and Development.

On returning to the University, Dr. Gillespie served for ten years as Director of the Great Lakes Center for Truck and Transit Research. His teaching included automotive engineering, vehicle dynamics, and vehicle design offered to university students and industry engineers. He retired from the University in 2006. He retired from his software company, Mechanical Simulation Corporation, in 2021.