When you have a problem with setting up your Gmail account to remail, please check the following settings:
First of All please Enable IMAP
In your Gmail account settings, make sure that IMAP is enabled here:

Please Enable Less Secure Apps and don’t worry
Switch on the access to less secure apps here. And be sure all is OK.👍
About the 2 Step Verification
Provided that you have two-step verification, you will need to generate a password for Remail, manually and connect an email account and enter the generated password in the settings of this email account.
To add your email account manually, go to Settings – > Email Accounts, then click the blue plus and enter the data instead of Gmail(or other E-providers).
Then manually enter all the settings for IMAP and SMTP:
Email address and sender’s name are required *
IMAP host: imap.gmail.com
port: 993
SSL enabled
Password: generated password
SMTP host: smtp.gmail.com
port: 465
SSL enabled
Password: generated password