Are you having a similar error when trying to send emails using nodemailer with your Gmail credentials ?
Error: Invalid login: 534-5.7.14 Please log in via your web browser and then try again. Learn more at https://support.google.com/mail/answer/78754 - gsmtp
The good news is your credentials should be ok, otherwise you would receive an error like this :
Error: Invalid login: 535-5.7.8 Username and Password not accepted.
The explanation can be found here : https://support.google.com/accounts/answer/6010255?hl=en, basically Google consider your app being a "less secure app" and blocks it.
The solution is to go on that page : https://myaccount.google.com/lesssecureapps and unlock "less secure apps".
Important note ! for some reasons, when I reload that page my choice seems to have disapeared, I had to re-unlock them again for this change to remain active.