Man pages matching name gnutls_cipher_get_block_size in section 3:

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