since iOS 9 installation (I'm on 9.0.2 now) I am not able to cancel notifications from the Notifications Center. They are mostly Gmail notifications, but not only that. The X is present, when I press it CANCEL appears , but it doesn't work. Even if I cancel the mail from Gmail app the notification stay there. Anyone found a solution?
it is working fine on my 5s on all the ios 9 releases. Have you tried toggling "show in notification center" or "allow notifications". Also, if that fails maybe remove you account from gmail app and delete and re-install the gmail app?