To optimize a mobile GUI for different device orientations, follow these steps:

1. Design for flexibility: Use flexible grids, layouts, and elements that adapt to changing screen dimensions.

2. Use responsive design: Implement responsive design techniques, such as media queries, to apply different styles based on device orientation.

3. Test and iterate: Test your design on various devices and orientations, making adjustments as needed.

4. Prioritize content: Ensure essential content is visible and accessible in both orientations.

5. Adaptable navigation: Design navigation that adapts to orientation changes, such as hiding or revealing elements.

6. Orientation-specific layouts: Create separate layouts for portrait and landscape orientations if necessary.

7. Consider aspect ratios: Ensure your design looks good in different aspect ratios, such as 16:9 or 4:3.

8. Use device-agnostic design: Avoid designing for specific devices, focusing on general screen sizes and orientations.

9. Leverage mobile-specific features: Utilize features like iOS's SwiftUI or Android's Jetpack Compose to simplify orientation handling.

10. Continuously test: Regularly test your design on various devices and orientations to ensure optimal performance.

By following these steps, you can create a mobile GUI that seamlessly adapts to different device orientations, providing an optimal user experience.