Man pages matching name gnutls_psk_server_get_username in section 3:

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