GST registration of stock broker

Under GST, registration is compulsory for persons who make taxable supplies on behalf of other persons as an agent.
You are compulsorily required to obtain registration under GST irrespective of your turnover.
It means that if an agent is making a taxable supply on the behalf of his principal, 
then he is compulsorily required to obtain registration under GST even if his turnover is less than Rs. 20 lakhs. 
Thus, if you work as an agent, then registration under GST is mandatorily required.