Please proivde a function gnutls_x509_crt_key_encrypt() which uses the subjectPublicKey to encrypt the given data.

gnutls_x509_crt_key_encrypt(gnutls_x509_crt_t cert,
                            const void * ptext, size_t ptext_len,
                            void * ctext, size_t ctext_len);

or a way to extract the public key so you can do it with the public key API.

This is needed to implement the MS-BRKP protocol in Samba with GnuTLS.