This bug was reported as app-crypt/gnupg-2.1.10: hkps support missing in
https://bugs.gentoo.org/show_bug.cgi?id=567822
It seems that configuring gnupg >=2.1.10 (not tested with lower version) with
--enable-gnutls and --disable-tofu results on
checking for LIBGNUTLS... no
configure: WARNING:
- Building without NTBTLS and GNUTLS - no TLS access to keyservers.
however, if --disable-tofu is not specified, or --enable-tofu is used instead,
gnutls is correctly detected and as such hkps support is built
checking for LIBGNUTLS... yes