What does Gamma Notional mean?

Gamma Notional is the sum of call gamma - sum of put gamma 

Why does Gamma Notional matter?

This allows you to estimate whether a stock has a larger call gamma position (if figure is positive) or put gamma position (if figure is negative). 

How to use Gamma Notional?

A large positive gamma position infers lower volatility in the underlying. A negative gamma position implies higher volatility in the underlying.