Testing a GUI for accessibility involves evaluating its usability by people with disabilities. Key aspects to focus on:

1. Color contrast and visual clarity

2. Keyboard navigation and screen reader compatibility

3. Clear and consistent navigation and controls

4. Alternative text for images and icons

5. Closed captions and audio descriptions for multimedia

6. Consistent and logical page structure

7. Sufficient font size and zooming capabilities

8. High contrast mode and night mode testing

9. Assistive technology compatibility (e.g., screen readers, braille displays)

10. User testing with participants with disabilities

Additionally, consider:

- Conducting automated accessibility testing using tools like WAVE, Lighthouse, or Axe

- Manual testing with assistive technologies

- Evaluating cognitive accessibility and clear language

- Ensuring consistent and predictable interactions

- Testing with different browsers, devices, and operating systems

- Documenting accessibility features and limitations

By thoroughly testing these aspects, you can ensure your GUI is accessible and usable for a wide range of users, including those with disabilities.