Both Sehwag and Gambhir are currently out of the squad owing to prolonged run drought, forcing the selectors to run out of patience.
Dhawan said he also observed Sachin Tendulkar batting at the nets, and could learn a few things by merely watching the senior batsman train.
Dhawan was given the news of his debut on Wednesday morning by coach Duncan Fletcher and captain Mahendra Singh Dhoni.
Talking about opening the innings with the Chennai right-hander, the left-handed Dhawan said coordination between the two will be very important if they are to form a successful partnership.
(with inputs from PTI)