GST on Commission agent?

As per section 24 CGST act ( compulsory registration)

vii] persons who make taxable supply of goods or services or both on behalf of other taxable persons whether as an agent or otherwise;

AND 

As per definition of agent
Section 2(5) of The Central Goods and Services Tax Act, 2017 provides for definition of agent as follows: “agents” means a person including a factor, broker, commission agent, arhatia, del credere agent, or 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.

If Turnover is less than 20L then registration under GST is not required as per sec 22 of CGST Act. But ensure all other supplies including exempted supplies are also to be included for 20L.