A GNU toolchain developer works on the GNU toolchain, a suite of software development tools including the compiler, linker, assembler, libraries, and debugger. The developer's responsibilities involve creating, maintaining, and enhancing these tools to improve their efficiency and functionality. They work on source code, fix bugs, and collaborate with other developers to integrate the tools into larger systems. They also assist in porting the toolchain to new platforms. Their goal is to ensure the toolchain provides a robust environment for software development, enabling developers to compile, debug, and optimize their code effectively and efficiently.