I would be right there with you except for the glaring lack of help in finding GP when she was missing. I can't get past that. I can't come up with any reason they would not help that doesn't make them look like the worst people on the planet, except that they knew that nothing they could add could save her. I am a pretty imaginative dude, I can usually find scenarios to support either side of a question, but I can't come up with one here for putting up with what they had been putting up with just to keep from talking about a break-up.
I am a bit behind on the thread but I can think and identify many reasons (only to counter this, not as any sort of determination or personal belief).
Note: I am also a hard core introvert by nature, so this is through my lens only.
1. They may have never engaged in a conversation before ever with Gabby's parents and had heard too many stories from Gabby about histories involved to engage now.
2. They may have simply not even seen the messages and calls until they became threatening with the police. It appears that Brian didn't much care for regular phone use, maybe they don't either, on their personal lines and when they looked, they knew nothing good would come of it and turned the communications over to the authorities and their attorney.
3. Maybe they knew that whoever Brian left Gabby with, IF he did-big IF, was/had been involved in something not so good and they wanted nothing to do with such a thing and didn't want to be the ones to tell her parents. Legal reasons.
4. Maybe they just have a rule about allowing toxicity into their lives and by the time they heard the messages and saw the texts, they decided it might be best to have the authorities handle it, instead of stepping into a fire that had begun to build.
5. There are additional theories that are possible, some even probable but, for very good reasons, are not able to be debated here due to TOS.
6. Final thoughts, we simply don't know what we don't know. So we are left with only assumptions and our own experiences to fill in the blanks.
There may be a GREAT reason that they made these choices. There may be NO reason (unlikely, but quite possible) and that would be terribly difficult to reconcile.
Please know that I only share these as possibilities on this side of the scale since that is what was mentioned. I very much agree that the opposite may also be true and deservedly confounding.
MOO!