A GXT developer is responsible for designing and implementing web applications using GXT (formerly known as GWT-Ext), a framework for building rich internet applications with Java. This involves creating user interfaces, writing efficient and reusable code, and integrating backend services. They also work closely with other developers to ensure system consistency and improve user experience. Debugging applications and resolving issues are also part of their job. They need to stay updated with emerging technologies and apply them into operations and activities. Furthermore, they often communicate with clients to understand their requirements and provide appropriate solutions.