The first mistake made by email clients is they added support for a broken-by-design protocol called S/MIME which used asymmetric encryption through the entire message and was thus cripplingly slow. The ciphers were also covered by patents and had weak key lengths. Messages were signed with a cert like https, and were required to be signed by a CA. And you couldn't get a key unless you paid a CA for one. Oh and keys expired meaning you might have multiple dead keys to maintain if you wanted to open an old email. And no email client or ISP actually offered to give you a key or set you up with one so you had to figure this all out for yourself. And functionality like search / filtering broke on encrypted mail because the client never bothered to maintain an encrypted index of the plaintext that could have allowed it to work.
Then PGP / GPG solved a lot of this bullshit, starting with generating keys for free but email clients never bothered to give it proper support. Instead they offered up some plugin APIs and unsurprisingly PGP / GPG ended up with half assed implementations too. Even fairly good extensions like Enigmail didn't integrate with the client as closely as they should.
And by this point cloud based email took off and crypto fell by the way side. If you want to use crypto in GMail then you have to cut and paste and clearly it's too much effort.
So I really don't blame GPG here. If the first thing an email did during setup was ENCOURAGE a user to create a key; and by default published that key; and attached the key sig to outgoing emails; and automatically looked up incoming email addresses; and automatically encrypted content when all recipients had their own key; and didn't hobble functionality for any of this (e.g. search still worked). THEN this wouldn't even be a problem. Encryption would have been the default and it would be an irrelevance if it was PGP or GPG was under the covers.