A GNU Make developer designs and implements build automation systems using GNU Make, a tool that automatically builds executable programs and libraries from source code. They write makefiles, which contain rules to derive files from other files. These rules instruct Make on how to derive the target file from the source file. The developer manages dependencies to ensure that changes in source files trigger the necessary updates in derived files. They also maintain and debug the makefile, ensuring it works across different platforms. Their role is critical in software development, providing an efficient and reliable method for managing complex projects.