I tried to use email encryption some years ago - not pgp (or gpg) but s/mime. I was involved with Thawte's 'Web of Trust' which verified identities so that cetificates could have trusted names. The problem then, and it's probably worse now, was that fewer and fewer people were using email clients that supported it; webmail was becoming the dominant method of reading and sending email and there were no webmail systems that supported it. I don't know about email apps on phones and tablets; my Android email client supports s/mime but I'm not sure my keys are secure on it.

To be convenient for non-geeks encryption needs to be built into the email client. Several clients support s/mime and for gpg and there is the enigmail addon for Thunderbird. But I suspect most do not.

I do use gpg, though not for messaging. For example I have a password file on this Android device which is symmetrically encrypted with gpg. Works under any operating system. Even iOS!