Frustrations over getting Graviga in chatper five

[Milkpea]

I'm doing a complete run through using the Brady guide book but I've come to a point that is totally baffling me. I'm currently in chapter 5 having just got onto the highway where Genesis is attacking the Shinra building. In the book it says to use Graviga on the G Assaliants. I look up where to gain this and it says missions 1-3-5,6-6-2,7-3-6,7-6-6,8-5-6. What's puzzling me is most of these missions ask you to use Graviga on the enemies.
What am I missing? @_@
Thank you

(edit: This is for Crisis core. I forgot to say that in the title)