applicability in case of commission agents?? and defination of turnover is the amount of commission earned or value of goods supplied???

respected sir,
under gst if adat i.e.commission agent who is appoited by oil extraction company to purchase product called soyabean from local market of particular territory and supply to them.that agent will purchase that soyabean in the name of oil extraction company only.at the time of payment total bill amount is credited to that commission agents account and then that commission agent will pay all the parties from whom he purchased soyabean in that market on behalf of that oil extraction company.
1)GST is applicable or not???
2) if yes on which amount will be considered as turnover for the purpose of applicability as well as calculation of gst tax amount??