Multiple encryptions are the process of encrypting an already encrypted message one or more times, either using the same or a different algorithm. It is also known as cascade encryption, cascade ciphering, multiple encryption, and superencipherment. Superencryption refers to the outer-level encryption of multiple encryptions.
This is exactly what the One Time Pad DigitalBank Vault Encryption System does.
It can be safe to use Gmail, as long as you make sure that whatever you send on Google is encrypted is an unbreakable way.
This is how it looks like a DigitalBank Vault encrypted email on Gmail. No one can crack this one-time pad encryption.
Why take chances of being hacked and email content exposed, especially if it's top classified information? Do you want to end up like John Podesta and Hillary Clinton? Two not so responsible people that do not understand anything in cybersecurity.
DO NOT TRUST ANYONE. NOT EVEN ENCRYPTED EMAIL SERVICES. They create your encryption keys! They manage your encryption keys. They say they have zero knowledge of the content of your emails, but this is BS.