Registration is Compulsory and Voluntary in GST. COMPULSORY: 1. When your turnover exceeds 20 lakh at anytime in the previous financial year then within 30 days you are required to be registered under GST: 2. When you deal in Interstate Sales and Purchase means Outside the State you supply the goods and services. Voluntary: 3. When you desired to get Input Tax Credit on the Purchase which you had made earlier prior to the Sales. And under voluntary registration under gst you are required to pay GST and also eligible for input tax credit at the same time.