Gmail, like all other email service providers is strongly committed to protecting its users from suspicious activities because of password theft. Google alerts you when your Gmail credentials have been used by an unknown application. So when you try to connect your Gmail account with Freshsales, you receive an email from Google with the subject Sign-in attempt prevented.

If you have enabled two-factor authentication, grant Freshsales access to your gmail account by generating an app specific password. If you haven’t enabled  two factor authentication, you can  turn on access for less secure apps.