How will GST affect taxes on trading income?

1.If my only source of income is from stock market trading, Do I have to register for GST?
2.How will this affect my income tax, will it still be based on tax-slab or do I need to pay extra 18%?

There are various types of services and supply provided by a broker as under:

Interest or Delayed Payment Charges

Also, GST would be application on any interest / delayed payment charges charged for delay in payment of brokerage amount / settlement obligations / margin trading facility.

GST for Sub-Brokers

Sub-brokers, as defined under Stock Brokers and Sub Brokers Regulation, 1992, means any person, not being a member of stock exchange, who acts on behalf of a stock broker as an agent or otherwise for assisting the investors in buying, selling or dealing in securities through such stock brokers.

As per section 2 (5) of CGST Act, 2017, the term “agent” means a person, including a factor, broker, commission agent, arhatia, del credere agent, an auctioneer or any other mercantile agent, by whatever name called, who carries on the business of supply or receipt of goods or services or both on behalf of another. Sub-broker would apparently be providing services to the stock broker as well as to the clients / investor and receiving consideration from both. Thus, in such a case, where the sub broker is providing services both to the broker and the investor on behalf of the broker, the sub broker would be duly covered within the definition of ‘agent’ as provided in Section 2(5) of the CGST Act, and needs to compulsorily register themselves under GST without the threshold exemption under Section 24(vii) of the CGST Act, 2017. However, if the sub-brokers do not provide any service to the clients / investor on behalf of stock broker (for example referral commission only), then the said sub-brokers would not fall in the definition of “agent” under the CGST Act, 2017 and provisions of GST would be made applicable accordingly.