Call gamma is how much dealer gamma is modeled to be on calls, such as a highly concentrated call strike. Gamma can be thought of as the acceleration of directional exposure. 

By saying dealer gamma, we mean gamma on market maker books. This is what is meant by call gamma as a metric. Heavy positive gamma on dealer books, especially concentrated around a particular strike, is known to create sticky pins (keeping the market in a tight range). 

The mechanism for this is that increasing hedging demands (a need to rebalance and therefore neutralize risks) for dealers causes them to trade rapidly in a way that can slow down or reverse a price trend, trapping in it a range with a decently strong probability depending on how strong that gamma is. This in turn creates credit spread (premium selling) opportunities, where there is a payout if the price does not move much further than these boundaries defined by high gamma. 

Call Gamma is one of the many sortable properties on the History tab of our Equity Hub™, which can be a useful sorting feature:

To understand this with more detail, SpotGamma refers to call gamma as the sum of all dealer call gamma for all available strikes. 

On the indices, we model call gamma to be positive and put gamma to be negative. The total market gamma, being the difference of total positive [call] gamma and total negative [put] gamma, is reported each day in the Founder’s Notes as the SpotGamma Gamma Index, which ranks market gamma positivity on a scale from -4 to +4.