To set up Rent It Biz to connect to the Gmail server, go to you Gmail account and select My Account.
Click on Gear in upper right hand corner then See All Setting 
Go To ACCOUNTS AND IMPORT Then OTHER GOOGLE ACCOUNT SETTINGS
Go To SECURITY on left hand side
Scroll Down to LESS SECURE APP ACCESS.
Allow Less Secure Apps: ON
Ensure that the following SMTP settings are used:
1. SMTP Server: smtp.gmail.com
2. SMTP Port: 587 **Google has at least 3 SMTP Ports. If 587 does not work, try 465 or 25. You may have to
test until you find the correct one
3. Make sure the password in RIB Mail Settings matches your password in your Gmail Account