Man pages matching name gnutls_certificate_set_verify_function in section 3:

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