A GWT (Google Web Toolkit) developer is responsible for designing, developing, and testing web applications using the GWT framework. They write complex Java codes that are converted into JavaScript, enabling the application to run in any browser. They also ensure the application's compatibility across different browsers and platforms. GWT developers optimize the application for performance, troubleshoot and resolve issues, and provide technical support. Additionally, they may collaborate with UI/UX designers to create user-friendly interfaces and with backend developers to integrate the application with server-side logic. They also need to stay updated with the latest GWT trends and technologies.