The Accelerator (Delta's Engine)
Gamma tells you how fast your Delta changes. It is the "acceleration" of your velocity.
Delta isn't static. As a stock moves, the Delta moves too. Gamma is what pushes Delta toward 1.00 (as it goes deep ITM) or toward 0.00 (as it goes deep OTM).
This is why "Lotto" trades (OTM options near expiration) can suddenly explode. On expiration day, Gamma is at its peak. A $0.10 move in the stock can cause an option's Delta to jump from 0.10 to 0.60 in seconds.
If you are short an option, Gamma is the "risk of the unknown." It makes your position much more sensitive to small price changes. This is why experienced traders close short positions before expiration week.
Near expiration, stocks often "pin" to strike prices due to market maker hedging. If you're short an option right at the strike, you face maximum uncertainty about whether you'll be assigned.
"Negative Gamma" is the main reason to close short-term income trades early. If you've made most of your profit, don't stay in the trade during the final 48 hours. The "Gamma Risk" (the potential for a tiny move to wipe out your gains) far outweighs the tiny bit of Theta left to collect.
Some traders intentionally buy options with high gamma and "scalp" the underlying stock as delta changes. This is an advanced strategy that profits from large price swings.