A fee is now required for the latest release of GPG Mail: GPG Mail Update Surprises Mac Users with Paid Plan September 25, 2018 https://www.bleepingcomputer.com/news/security/gpg-mail-update-surprises-mac-users-with-paid-plan/
As I recall, one can install standard GnuPG with brew. And maybe the standard Thunderbird+Enigmail setup also works on macOS.