Man pages matching name gnutls_x509_trust_list_get_issuer_by_dn in section 3:

gnutls_x509_trust_list_get_issuer_by_dn.3.en	debian 10 / gnutls-doc
gnutls_x509_trust_list_get_issuer_by_dn.3.en	debian 11 / gnutls-doc
gnutls_x509_trust_list_get_issuer_by_dn.3.en	debian 12 / gnutls-doc