GB married AM's sister back in 1984, when AM was only 9 years old. GB's eldest daughter is AM's niece. If AM was willing to treat JB's girls as more nieces, I think that GB and JB would probably have appreciated that (it's never good for one child in a family to be singled out for extra attention).
I think it can be really difficult to see through someone you've known since they were a child that young.
AM strikes me as the slick abuser type; someone who was very careful to keep the abuse hidden. Abusers like that are invariably very well controlled and very good at putting up a facade.
Considering the difference in age and the reports that the two men enjoyed working on cars together, I wonder if GB was the one who taught AM how to work on cars.
Also, it has been reported that AM never held a regular job but relied on odd jobs. GB may have felt sorry for him and wanted to give him some help.
If so, then I think it would be even more difficult to see through this brother-in-law and protege.
There have certainly been lots of reports from neighbours and casual acquaintances saying AM seemed like a nice guy, very helpful, etc. GB would not have been alone in being fooled.