NTBTLS
`Not Too Bad TLS`
is a TLS client-only library developed by Werner Koch.
- It started (2014) as a stripped down fork of PolarSSL (now called mbed tls),
- using libgcrypt and existing GnuPG modules for X.509 certificate handling.
https://git.gnupg.org/cgi-bin/gitweb.cgi?p=ntbtls.git;a=summary
Rationale
TLS client connections are needed in GnuPG for transport of certificate, pubkeys and revocation information. Using gnutls, mbed_tls or a different TLS library would introduce additional crypto code which makes the whole system harder to evaluate. GnuPG already has a number of the required technical functionality.