To protect a GUI application from unauthorized access, consider the following measures:

1. Authentication: Implement robust authentication mechanisms, such as username/password, two-factor authentication, or biometric authentication.

2. Access Control: Enforce role-based access control, limiting features and data access to authorized users.

3. Encryption: Use encryption to protect sensitive data, both in transit and at rest.

4. Secure Communication Protocols: Use secure communication protocols, such as HTTPS or SFTP, to protect data in transit.

5. Input Validation: Validate user input to prevent SQL injection, cross-site scripting (XSS), and other attacks.

6. Secure Password Storage: Store passwords securely using hashing and salting.

7. Session Management: Implement secure session management, including secure session IDs and timeouts.

8. Regular Updates and Patching: Regularly update and patch the application to address security vulnerabilities.

9. Secure Configuration: Implement secure configuration options, such as disabling unnecessary features.

10. Monitoring and Logging: Monitor and log security-related events to detect and respond to potential security incidents.

11. Network Segmentation: Segment the network to limit access to sensitive areas.

12. Firewall and Intrusion Detection/Prevention Systems: Use firewalls and intrusion detection/prevention systems to block unauthorized access.

13. Secure Deployment: Securely deploy the application, including secure installation and configuration.

14. User Education: Educate users on security best practices and the importance of protecting sensitive data.

By implementing these measures, you can effectively protect your GUI application from unauthorized access and ensure the security and integrity of your users' data.