I purchased ETH on GDAX exchange. Since I am a novice to blockchain world, I wanted to look up the transaction on Etherscan. Tried entering account id, transaction id and can't find anything.
What am i doing wrong ?
Usually operations within an exchange are not sent to the blockchain, they are internal operations within the exchange. An exchange allows the user to extract their coins to an external account paying a fee, that operation will generate a transaction in the blockchain.
As of August 2017 both Gdax and Coinbase are part of the same group Coinbase vs GDAX. So it is possible they have an arrangment if you transfer between them you do not have to pay fees, that means no transaction is generated in the blockchain.