If you cannot add your Gmail email account to StealthMail application with a valid credentials you have to do the following:
Sign in to your Gmail account.
Click on the Security tab (left side of the page).
Scroll down to the Less secure app access block.
Turn ON Allows less secure apps.
Go back to Security page by clicking on back arrow.
Scroll down to 2-Step Verification and click on it.
Follow instructions to enable 2-Step Verification.
Go back to Security page by clicking on back arrow.
Scroll down to App passwords and click on it.
If prompted enter your password again. This is an additional security check.
In the appeared form pick Other item in the first drop down.
Enter StealthMail in the appeared input control (or enter any other name you like).
You will get a generated password. Copy and save it for later use.
Enter your email account name and saved password.
In a while your Email account should be successfully added.
IMAP incoming server: imap.gmail.com
IMAP port: 993
IMAP SSL / TLS: TLS
SMTP outgoing server: smtp.gmail.com
SMTP port: 465
SMTP SSL / TLS: TLS