We strongly advise using SmoothPay's zero-configuration SecureMail service, however the option to use Google smtp servers for email delivery is available should you wish to use it.
Google continually change or add new security features that can easily break third-party applications ability to send emails via their smtp service (such as SmoothPay).
However, a few minutes setup is all that is required to make sure SmoothPay is able to continue to use Google's email service for payslip delivery etc.
You can generate an app password from your Google account (login here)
Then select App passwords (in the Signing in to Google section of your account)
This is where you add SmoothPay as a custom application and generate an APP password that can be used in your email settings in SmoothPay.