Gmail now has a new feature, "Confidential Mode". The Electronic Frontier Foundation is (rightly) skeptical of how "Confidential" it is. Gmail seems more interested in limiting the actions the intended recipient can take than in ensuring that it is in fact received by the intended recipient.
Yes, the name is really misleading! There's no guarantee of confidentiality here, unless your emails can only be accessed on a completely secured system.
The idea of having google send pass-codes via SMS is also weird, since you may not always have access to a mobile number of the party you're emailing, and even if you do, they may not want their privacy violated.
It would make more sense to email info in an encrypted format & then convey the pass-code to the recipient via a separate channel --- without Google involved in the latter.