Designing a GUI that complements a voice-first interface involves:

1. Visual minimalism: Simple, clean design to avoid visual overload.

2. Contextual visual cues: Visual hints to support voice commands.

3. Consistent layout: Logical layout for easy navigation.

4. High-contrast colors: Clear visual distinction between elements.

5. Prominent feedback: Clear visual feedback for voice commands.

6. Complementary functionality: GUI provides additional functionality.

7. Redundancy reduction: Minimize duplicate voice and GUI functionality.

8. User guidance: Intuitive guidance for using both interfaces.

9. Adaptive design: GUI adapts to user behavior and preferences.

10. Multimodal interaction: Seamless switching between voice and GUI.

11. Feedback synchronization: Sync voice and visual feedback.

12. Error handling: Clear visual error handling for voice commands.

13. Accessibility: Ensure equal accessibility for users with disabilities.

14. Aesthetic consistency: Consistent visual style throughout.

15. User testing: Test with users to refine the design.

By following these principles, you can create a GUI that effectively complements a voice-first interface, providing a harmonious and intuitive user experience.