Indeed disappointing -- this has to be the single most requested feature in Gmail, how can they not open a new universe which could allow this, and then not include this.
Not only that, but they have a guy who AFAIU spent his 20% time to build this feature (i.e. e2ee using GPG), it would take minimal resources to implement it as an add-on. I guess with all the usage data they have they know how much the general public cares about privacy. It's sad not even the Googlers who care can push some of these "higher good" features without being reassigned to some mundane shit.
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:-/