libprelude build-depends on libgcrypt20-dev (and libprelude-dev depends
on it) but does not use libgcrypt.

sid)ametzler@argenau:/dev/shm/GCRY/libprelude-5.2.0$ grep -rl gcrypt
NEWS
ChangeLog
debian/changelog
debian/control
debian/libprelude-dev/DEBIAN/control

Quote from NEWS:
- Get rid of libgcrypt

    Nowadays, libgcrypt functions that we used are now available
    directly from GnuTLS, which uses libnettle.

cu Andreas