The most popular GUI development tools for mobile applications are:

1. Flutter (open-source, developed by Google)

2. React Native (open-source, developed by Facebook)

3. Xamarin (proprietary, developed by Microsoft)

4. Ionic (open-source, developed by Ionic Framework)

5. Android Studio (proprietary, developed by Google)

6. Xcode (proprietary, developed by Apple)

7. Adobe XD (proprietary, developed by Adobe)

8. Figma (proprietary, developed by Figma)

9. Sketch (proprietary, developed by Sketch)

10. Qt (open-source, developed by The Qt Company)

These tools allow developers to create native or cross-platform mobile applications with a focus on GUI design and development. They offer various features like drag-and-drop interfaces, visual editors, and code generation to streamline the development process.

Flutter and React Native are popular choices for cross-platform development, while Xamarin is widely used for enterprise mobile applications. Android Studio and Xcode are the official integrated development environments (IDEs) for Android and iOS development, respectively.

Adobe XD, Figma, and Sketch are popular design tools used for creating mobile app prototypes and GUI designs before implementing them in code. Qt is a cross-platform framework that allows developers to create GUI applications for mobile, desktop, and embedded devices.