Man pages matching name gnutls_x509_privkey_cpy in section 3:

gnutls_x509_privkey_cpy.3.en	debian 10 / gnutls-doc
gnutls_x509_privkey_cpy.3.en	debian 11 / gnutls-doc
gnutls_x509_privkey_cpy.3.en	debian 12 / gnutls-doc