Resolve Gmail SMTP Authentication Error

I'm experiencing an issue with sending emails using Gmail SMTP. The error message indicates a failure in authentication due to incorrect credentials. I need assistance in resolving this Swift_TransportException error in AbstractSmtpTransport.php line 383, which states: "Expected response code 250 but got code '535', with message '535-5.7.8 Username and Password not accepted.'"

Key Requirements:
- Expertise in configuring Gmail SMTP for email sending.
- Ability to troubleshoot and resolve authentication errors.
- Knowledge of Gmail's security settings and App Passwords.

Ideal Skills and Experience:
- Experience with email server configurations, particularly Gmail SMTP.
- Familiarity with two-factor authentication (2FA) and App Passwords.
- Strong problem-solving skills to diagnose and fix credential issues.

I have two-factor authentication enabled on my Gmail account but have not created an App Password for this task. I need guidance on setting up an App Password and ensuring proper configuration to successfully send emails.