Undocumented in source but is binding to an external library. You might be able to learn more by searching the web for its name.
intgnutls_openpgp_privkey_export_rsa_raw(gnutls_openpgp_privkey_t pkey, gnutls_datum_t* m, gnutls_datum_t* e, gnutls_datum_t* d, gnutls_datum_t* p, gnutls_datum_t* q, gnutls_datum_t* u)