In order to use a Google account as your SMTP provider, you will first need to complete the following steps:
1. Turn OFF 2-Step Verification
Sign in at accounts.google.com, select Security from the left sidebar, then turn off 2-Step Verification
2. Turn ON Less secure apps
Go to Manage your Google Account:
Go to Security-->Turn on less secure app access
If you don't see the Less secure app access section, it is because Access to less secure apps is disabled
Please sign in to admin.google.com
From the Admin console Home page, go to Security
Less secure apps.
Select "Allow users to manage their access to less secure apps"
Hit Save
After you switch it to allow users to manage their access to less secure app. You still need to go back to Manage your Google Account
Go to Security-->Turn on less secure app access
3. Unlock captcha
Try clearing Captcha: visit https://accounts.google.com/DisplayUnlockCaptcha and sign in with the Gmail username and password. If necessary (it's usually not), enter the letters in the distorted picture then press Continue. This will allow ten minutes for the 3rd party account to register as an approved connection. Note that you must use the account you are trying to add to the 3rd party account - if the browser is already signed into another account, you must sign out first.