To use notifications effectively and keep users engaged with a GUI:

1. Make them relevant: Tailor notifications to user interests and actions.

2. Keep them timely: Send notifications at the right moment to maximize impact.

3. Be concise: Keep notifications brief and to the point.

4. Use clear language: Avoid jargon and ensure notifications are easy to understand.

5. Provide value: Ensure notifications offer value, such as updates, tips, or rewards.

6. Offer control: Allow users to customize or opt-out of notifications.

7. Use visual hierarchy: Use color, size, and position to draw attention to important notifications.

8. Avoid over-notifying: Limit notifications to avoid overwhelming users.

9. Use different channels: Combine GUI notifications with email, mobile, or in-app notifications.

10. Monitor and adjust: Analyze user response and adjust notification strategies accordingly.

Effective notification types include:

- In-app notifications

- Push notifications

- Email notifications

- Toast notifications

- Pop-up notifications

- Badge notifications

By using notifications thoughtfully, you can re-engage users, encourage desired behaviors, and enhance the overall GUI experience.