Person engaged in 100% export of services, is required GST registration if the turnover exceed Rs.20 Lakh.

Reply—Under section 22(1) Every supplier shall be liable to be registered under this Act in the State or Union territory, other than special category States, from where he makes a taxable supply of goods or services or both, if his aggregate turnover in a financial year exceeds twenty lakh rupees.

However,  persons making any inter-state taxable supply are required compulsory registration under section 24 of the Act. Export of services are also covered under inter-state supply.

Therefore, registration is required. Export is treated as inter-state supply.