Gay Stewart received her Ph.D. in particle physics from the University of Illinois, Urbana-Champaign in 1994. In 1993, she attended a conference on undergraduate faculty enhancement and became interested in physics education research. She accepted a faculty position at University of Arkansas in 1994, where she and her collaborator John Stewart focused on three primary interrelated issues: improving the introductory sequence to better prepare students to succeed in science and engineering degrees, improving the preparation of physics majors for the variety of career options open to physicists, and the preparation of future faculty, for both the high school level and the professoriate. The undergraduate program at UA saw dramatic improvement, with a 10-fold increase in number of graduates, with many receiving national awards. UA was one of six initial primary program institutions of the Physics Teacher Education Coalition (PhysTEC).

Gay first received NSF support for her work in 1995. In 2014, Gay transitioned to West Virginia University, where she is the Eberly Professor of STEM Education, professor of Physics and director of the WVU Center for Excellence in STEM Education. She is PI of one of the first INCLUDES Alliances and project director for WVU’s HHMI Inclusive Excellence 3 work among other efforts. Collaborator John Stewart leads WVU's STEM retention research and PhysTEC implementation.

Gay is a Past President of the American Association of Physics Teachers (AAPT). She is a fellow of the APS and AAPT and served as the councilor representing the Forum on Education and also on the APS Board of Directors. She previously served on the APS Forum on Education Executive Committee, both as the AAPT liaison and in the chair line; and the APS Committee on Education. She currently serves on the Committee on Education. She chaired the College Board’s Science Academic Advisory Committee, co-chaired the Advanced Placement Physics Redesign commission, and the AP Physics 2 Development Committee. Gay was chosen the 2002 CASE Arkansas Professor of the Year and has received numerous awards for teaching and advising.