Actually, lots of OTTBs have impeccable ground manners. Depends on who broke them and how they were handled at the track. Remember, an OTTB has been physically handled more and done more for their age than almost any other type of horse. As long as that handling was kind and non-abusive, the ground manners shouldn't be an issue. If you get an horse that went through a yearling sale, their ground manners are even better.
Since your grandfather trains, and will be picking the horse for you, I'm sure he'll pick something suitable to reclaim. I'm glad to hear it will be a gelding. I've had some issues with OTTBs that were gelded late, so I'm hoping this horse will have been gelded fairly young.
The lay up/let down time is important. Lots of turn out time if you can manage it.
Two excellent books: After the Track, by Susan Ford and Reschooling the Throroughbred Racehorse, by Peggy Jett Pittenger. Second one is somewhat dated, but still a useful reference.
Be kind, go slow, have fun.