A GNU C developer designs, codes, and debugs applications using the GNU C programming language. They work on the GNU operating system, which is a Unix-like, free software operating system. They are responsible for writing system software, libraries, and compilers, ensuring the efficient execution of programs. They also maintain and update existing code, fix bugs, and improve system performance. They collaborate with other developers, participate in code reviews, and adhere to software development methodologies. They must have a deep understanding of computer architecture, data structures, and algorithms, and stay updated with the latest programming techniques.