This is how they have defined notification under GST
(80) notification means a notification published in the Official Gazette and the expressions notify and notified shall be construed accordingly.
So, for GST the existence of notification is from the date of publication under official gazette.
Gazette is a public journal. Government has to publish such law so that they may say that ‘ignorance of law is not an excuse’. If something doesn’t get publish in the gazette and just there on website as notification comprising of effective date. It would be very contrary to the very basic right of the general public knowing the government notice, law etc via gazette.