Government mandate GST Registration for businesses who fall in given criteria which are:
Businesses with turnover exceeding ₹40 lakh (₹20 lakh for service providers)
Inter-state suppliers of goods or services, inter state supply means sale of goods or services in different state.
Doing business through E-commerce platform like Flipkart, Amazon, India Mart, etc or aggregator platforms
Doing occasionally business in India like selling goods or services through event held in India
Business by Non-resident taxable persons in India.
Input Service Distributors (ISD)
If you are person acting as an agent and supplying good or services on behalf of your principal. In this case GST Registration is compulsory irrespective of turnover.
Also, the registration process is totally online, it will take around 10-15 working days to get the approval.