Presumably with the web client. There is nothing to prevent sending and receiving encrypted/signed emails to/with a capable MUA using a gmail-based address (I know this because I have done it).

The gap in the market is accessing those emails via the web interface (or native gmail phone apps).

Your implication that they don’t add this capability because it is at odds with their values or what brings them value is at best only partly correct, and the most compelling reason for its being correct is that the support costs for average users - and the risks of having those users make errors and get the security wrong, which is entirely a user error in the current space - is simply too costly for them to gain value from supporting the capability.