The most common issues I have seen with gmail is
1. Google does not recognize the connection being used for the particular email account being used. The best thing to do is to go to their Login Activity
page and tell them basically that "yes, that was me"
Sign in - Google Accounts
2. Google by default blocks SMTP access under the premise that it is less secure. There is an option to allow "Less Secure Apps" which will allow SMTP access
Account settings: Your browser is not supported.