Necro-post but I found this because my game "glitched".
For the record, if you go to Grace early on before the John Seed story progresses you can get her as a companion early on. At some point during John's main story you cannot unlock Grace until after John is beaten. I actually unlocked Grace before Nick in a previous save game, before I ever started John's story.
Just wanted to clear this up for anyone else that comes on this post for the same search of a "Grace glitch".