Draft paper linked from efail.de.
TL;DR: the vulnerability is in some popular email client software combined with an extension simplifying use of GnuPG within said software, and bundled in popular distributions of GnuPG v2, thus often used.
Disabling GnuPG functionality in the email client and getting back to manual use of GnuPG should be fine. I would do that, rather than trust me and an email program not to follow links in received emails, which according to some reports is a necessary condition for the vulnerability to be exploitable.