Not available to users with personal Google Accounts.

Why not?

I’m speculating about how this works, but it probably doesn’t make sense for general-purpose email.

For emails between members of one workspace/domain, you can conceptually perform key exchange to implement client-side encryption. You also don’t have spam concerns within members of a workspace.

There’s no general way to do key exchange between different email domains, and you do have spam concerns (which to address requires scanning the content).

There’s no protocol for doing such things over different email domains in a way that’s compatible and interoperable across software stacks. This technology is proprietary, and would make spam challenging to deal with (which again is not a problem within one workspace).

It’s also not clear to me whether this is really client-side encryption, so much as encryption at a layer higher than the email stack. (It’s pretty hard to do client-side encryption in the browser, in a way that matches user expectations. How do you store/retrieve the encryption key?)

A number of Chrome (and I think also Firefox) extensions include their own local copy of OpenPGP.js for use with various webmail services, including GMail.

WKD (and HKP) depends upon HTTPS without cert pinning, FWIU: https://wiki.gnupg.org/WKD

  How does an email client use WKD?
  1. A user selects a recipient for an email.
  2. The email client uses the domain part of the email address to construct which server to ask.
  3. HTTPS is used to get the current public key.
  The email client is ready to encrypt and send now.

  An example: 
  https://intevation.de/.well-known/openpgpkey/hu/it5sewh54rxz33fwmr8u6dy4bbz8itz4 is the direct method URL for "[email protected]