Why are we assuming that G is always with Lisa

The video explanation for question 3 states that since "G is always with Lisa," but L is the necessary condition, not the sufficient one. So is it not possible to have L without G? I understand it might not be possible to do that once you factor in the other conditions, but I'm confused about how or why the video explanations assume that G must be in if L is in. my diagram: G --> L // not L --> not G