Man pages matching name gnutls_x509_crt_get_authority_key_gn_serial in section 3:

gnutls_x509_crt_get_authority_key_gn_serial.3.en	debian 10 / gnutls-doc
gnutls_x509_crt_get_authority_key_gn_serial.3.en	debian 11 / gnutls-doc
gnutls_x509_crt_get_authority_key_gn_serial.3.en	debian 12 / gnutls-doc