gnupg (2.x) should be coexistent with gnupg1 (1.x)
Some applications require the behaviors of gnupg1 (duplicity - even though it is getting updated it sounds like) while in general it is good to also keep around the newer gnupg series for its newer features.
For some reason installing gnupg1 will cause a purge of gnupg, likewise trying to re-install gnupg 2.1.5 will cause either nothing to happen (apk add gnupg) or unsatisfiable constraints:
# apk add -s "gnupg>=2" ERROR: unsatisfiable constraints: gnupg1-1.4.18-r0: breaks: world[gnupg>=2] satisfies: world[gnupg1] gpgme-1.5.4-r0[gnupg] gpgme-1.5.4-r0[gnupg] duplicity-0.7.03-r0[gnupg] gpgme-1.5.4-r0[gnupg]
A reasonable fix it would seem is to have the gnupg package not provide a symlink from gpg =>gpg2 and anything requiring the ‘gpg’ executable add a requirement on gnupg1.
(from redmine: issue id 4447, created on 2015-07-13)