Even then, it may not be private. You need to be able to trust the device and the client software on both ends of the connection. Apple says they care about your privacy, which is why they were talking about putting mandatory client-side filtering on your device to try and control what you are sending and receiving even when your data is encrypted. When you talk about using encryption to protect data sent through Snapchat the app from Snapchat the company, you're putting the unencrypted data into Snapchat's app, so even if they say it is end to end encrypted, even if it really is end to end encrypted, you can't be sure they aren't reading it anyway.

E-mail can be private, even with gmail. When you send mail through Protonmail, it uses standard PGP encryption, at least for inter-server mail. If the recipient has a public key on file, even if the recipient is using gmail, the message will be sent end to end encrypted. You can use PGP with any mail service that isn't filtering for and blocking PGP.