Man pages matching name gnutls_alpn_get_selected_protocol in section 3:

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