Hi
You will be chargeable to GST @18% on your profit element i.e. the commission income from such transaction.
Example, if you buy a medicine for 100 from Turkey and sell it in US for 200, you need to pay GST @ 18% on 100 (200-100) I.e. 18.
Also you need to register under GST when you start this business. There's no threshold limit.
#this answer is based on the general dropshipping model. We may discuss further based on any changes in relation to invoicing, etc.