GnuTLS is the GNU SSL/TLS implementation, because OpenSSL's license is incompatible with the GPL. There are a number of small compatibility issues between the two, so it's best to use the OpenSSL tools to create certs and keys for use by OpenSSL-linked servers and the GnuTLS tools to create certs and keys for use by GnuTLS-linked servers. See X.509 certificates for details on creating self-signed keys with both packages.