Gerald Jay Sussman is a professor at MIT and a central figure of the mit-ai-lab during the mit-ai-lab-hacker-culture-1971-1984 era. He co-created the Scheme programming language with guy-steele, an influential dialect of Lisp that became central to computing education through the classic textbook "Structure and Interpretation of Computer Programs" (SICP).
Sussman was stallman's mentor at the MIT AI Lab, where Stallman worked as a programmer. The collaborative, open culture of the AI Lab — where software was freely shared and modified — shaped Stallman's understanding of what software development could and should be. When that culture began eroding under pressure from proprietary software, Stallman's response was the gnu-project and the free-software-definition.
Sussman represents the academic hacker tradition documented in hackers-levy by steven-levy: deep technical skill combined with an ethic of open inquiry and shared knowledge. The hacker-ethic-mit that Levy documented and that Stallman sought to preserve and codify was instantiated in figures like Sussman.
The MIT AI Lab context — including Sussman's influence — is covered in free-as-in-freedom-williams by sam-williams and provides essential background for understanding why Stallman founded the gnu-project as a response to proprietary enclosure of software.