According to GST law, Every Person is required to get registration under GST law if Turnover of its business exceeds Rs. 20 Lakh (in some specified states it is Rs. 10 Lakh).

The meaning of Turnover is expressly defined by GST law under Section 2 as follow:

Aggregate Turnover shall include all taxable supplies, exempt supplies, Exports of goods, interstate supplies. All these supplies shall be taken of the Same PAN holder on all India basis. All these supplies shall be taken into consideration exclusively of CGST, SGST, IGST, UTGST, and Cess.
Even Non-taxable Supplies shall also be taken into consideration while computing the limit of Rs. 20Lakh or 10 Lakh, as the case may be. Example of non-taxable supply is Alcohol for human consumption etc.
But if any person is supplying exempted goods exclusively, then such person is not required to get registration under GST law even if its turnover exceeds the specified limits.