Totally unpopular opinion but...
With all due respect and empathy to Gabby's parents, they assume a great deal without any facts, in this letter.
1. They assume their son has told them where he left Gabby.
If they're not responding to Gabby's family, and Brian isn't talking, how would they know this? At this point, literally no one knows if he's told them anything, and if he did, if it was the truth.
2. They accuse Brian's parents of not helping and putting them through this, keeping Gabby's location hidden and putting Gabby's younger siblings through this.
None of us, and that includes Gabby's family, know if Brian's parents know a thing about the details.
If they don't know anything, they can't help, and they're certainly not the ones putting Gabby's siblings through anything or keeping Gabby's location hidden.
The sole person responsible for her disappearance, is the one guilty of that.
And we still don't know who that person is.
3. They go as far as bashing their character by asking "if they have any decency left".
I completely get that they're hurting deeply, but trashing Brian's parents seems rather unfair and unhelpful, considering the abundant lack of info and details.
Now, if it turns out the parents of Brian did know details and refused to help, I'll eat this post and all the crows y'all can find.
jmo