Did you notice a difference in the 'speed' of each step of the reaction?

When you watched the movie you saw that the reaction

2G + B --> G2B

occurred in two steps. The 'speed' of each step was different. Once the reaction was initiated collisions between G atoms did not always produce a G2 molecule. Once a G2 molecule was formed any collision with a B atom produced a product. The difference in speed for each step is noted as shown below.

Step 1: G + G --> G2 (slow)

Step 2: G2 + B --> G2B (fast)