Is GST registration mandatory for online sale of courses

1. selling courses would most probably attract lower threshold limit ie 20 lakhs unless what you are selling constitutes "selling of goods". 

2. a lot of things come into play for gst registration when you are selling online such as payment gateway may ask for it.

It means we wont be requiring to register our self under GST since beginning and can sell our courses till thresh hold limit of 20 lakhs all across India. 

yes. the requirement for registration triggers at 20 lakhs turnover.