Gamma notional is the sum of call gamma minus the sum of put gamma. This communicates whether a stock is modeled to have a larger call gamma position (if the figure is positive) or put gamma position (if the figure is negative). Also, a positive gamma position infers lower realized volatility in the underlying stock, and a negative gamma position implies higher realized volatility in the underlying stock. 

In general, the meaning of gamma is an acceleration of directional exposure. When market makers have high levels of positive gamma on their books, then it can slow down breakout price trends and also promote a relatively smaller trading range.