https://wiki.debian.org/msmtp <== msmtp setup mentions creating an encrypted password file

But when you try the command you get an error

That is because you need to create a private and public key with gpg first. Then the gpg --encrypt ... command above will work without error

Note the --passphrase '' command creates an empty passphrase so you don't have to enter anything to unlock the gpg private key before it decrypts the encrypted file. Note: This empty passphrase is so you can run msmtp from the command line, which decrypts the Google app password without ever having to enter a password... effectively make the password easy to obtain.

Then you can create the encrypted password file:

Then list your keys

Notice one key has an expiry date and the other is blank (no expiry) where it would say the expiry date

Then delete one that you don't want

During the above delete you will be prompted to delete the key twice with a text screen as follows