To design a GUI navigable using only a keyboard:

1. Logical tab order: Establish a logical tab order for elements, following a consistent pattern.

2. Focus indicators: Provide clear visual focus indicators for keyboard navigation.

3. Keyboard-friendly controls: Use controls that are easily accessible via keyboard, such as buttons and links.

4. Access keys: Assign access keys (shortcut keys) to important elements.

5. Navigation menus: Implement keyboard-navigable menus, using arrow keys and Enter to select.

6. Screen reader compatibility: Ensure compatibility with screen readers for visually impaired users.

7. Consistent keyboard shortcuts: Use consistent keyboard shortcuts throughout the GUI.

8. Clear and consistent labeling: Label elements clearly and consistently for keyboard navigation.

9. Grouping and containers: Use grouping and container elements to organize content for keyboard navigation.

10. Testing: Thoroughly test the GUI with only a keyboard to ensure navigability.

Additionally, consider:

- Using ARIA attributes for dynamic content and custom controls

- Providing a "Skip Navigation" link for bypassing repetitive content

- Implementing a logical and consistent navigation flow

- Using keyboard-navigable dialog boxes and alerts

By following these guidelines, you can create a GUI that is easily navigable using only a keyboard, enhancing accessibility and usability.