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)