Because there is no published glycemic index (GI) for broccoli as with many low carb veggies (hard to test), an estimated GI of 50 was used (below the average GI for veggies as a whole that have been tested). You could argue that it could end up having an even lower GI, but since broccoli has so few total carbs, the FP would still be on the low end.
For instance, 1 cup chopped broccoli (91 grams) has 6 g total carbs (two grams are fiber) and a calculated FP of 4 grams. Even if the GI was zero, the FP would only go up to 6 grams. Of course that’s still enough carbs to result in two liters of gas if totally malabsorbed.
Having said that, if you have gas from broccoli, it might be best to go with your gut and limit or eliminate it.