A GUI (Graphical User Interface) testing developer is responsible for testing software applications to ensure their visual elements function properly. They design and execute test cases, scripts, and procedures to evaluate the usability, functionality, and consistency of the interface. They identify, analyze, and document any defects or inconsistencies and work closely with the development team to correct them. They also ensure that the software aligns with user expectations and standards. They use various testing tools and methodologies, such as automated testing, to optimize the process and ensure the reliability and efficiency of the software's graphical user interface.