Measuring the effectiveness of a GUI in terms of UX involves evaluating various aspects of user experience. Here are some key metrics and methods:

1. User satisfaction: Surveys, feedback forms, and ratings to gauge overall user satisfaction.

2. Task completion rate: Measure the percentage of users who successfully complete tasks.

3. Time on task: Record the time taken to complete tasks, aiming for shorter times.

4. Error rate: Track errors made during tasks, aiming for lower rates.

5. User engagement: Monitor metrics like click-through rates, bounce rates, and session duration.

6. Net Promoter Score (NPS): Ask users how likely they are to recommend the product.

7. Heuristic evaluation: Expert review of the GUI against established UX principles.

8. User testing: Observe real users interacting with the GUI, identifying pain points and areas for improvement.

9. A/B testing: Compare different GUI versions to determine which performs better.

10. Analytics tools: Utilize tools like Google Analytics to track user behavior and identify trends.

11. Cognitive walkthroughs: Evaluate the GUI's ease of use and intuitiveness.

12. User interviews: Conduct in-depth interviews to gather qualitative feedback.

By combining these metrics and methods, you can comprehensively assess the effectiveness of a GUI in terms of UX and identify areas for improvement.