GST Registration - Professional

I have a query with respect to GST registration. A professional's gross revenue is less than INR 20,00,000 from his profession, but his capital gains, interest and other income together with professional income exceeds INR 20 lakhs. Does he have to take GST registration in that case? Also capital gains and other income decrease/increase in different years, so the turnover will have impact in that year if total income is less than 20 lacs. Please share your views.