I was surprised that nobody actually showed you how to fix this but only that there was an issue and you have to turn on the Less than Secure setting.
You setup sending forms in Quickbooks preferences and set it for Gmail, and the setting are mostly automated if you select Gmail and it will fill in the rest. No need to change the port or the server.
Some people want to use Gmail to send forms out of Quickbooks as of this date May 24th, 2016.
Nothing works, no port will work with or without SSL checked. It is not a Quickbooks thing.
The issue lies with Gmail settings. Sometimes your gmail account has an option to make settings for "less than secure" and you can look in.
Upper right corner >> My Account >> Sign-In & Security >> Less than Secure
If in Your Account, and you do not see "Less than Secure" then your Gmail Administrator will need to make the setting for you.