Gamma: If Delta is the amount of change that the option will experience in response to a change in the underlying asset price, then Gamma is the amount you can expect Delta to change over time.
Gamma increases as options near the point where the price of the options and the price of the underlying asset overlap and will decrease even further below the strike price as the price of the underlying asset drops.
The larger the Gamma, the larger the risk, but also the larger the return.