First you compute the Gamma of a European call option with strike price 100, which matures in 1 year. This will define the Gamma as a function of the risk-free rate, the dividend yield, and the volatility.
In this example you will use numeric values for the risk-free rate, the dividend yield, and the volatility.
You can also use the generic method in which the option is defined through its payoff function.
Here are similar examples for the European put option.
In this example, you will compute the Gamma of a strangle.
Check: