In hitois, when a friend posts an article, that notification is sent by e-mail.
The mail arrives from the mail address "hitois Mail (firstname.lastname@example.org)".
However, GMail's spam filter is powerful, so notification mails may be treated as spam. In this case, since the notification mail does not arrive in the inbox, I think that the mail has not arrived, but in reality it is delivered to the junk mail box.
In order to prevent notification mails from being treated as spam, you need to register "email@example.com" as a contact in GMail's address book. Registration in the address book is possible on the page of the following URL.
Go to this page and add "firstname.lastname@example.org" with "Add new contact". After adding, it will not be treated as spam mail.