I'm disappointed in the lack of rampant speculation in this thread, ha ha! Why are people being so objective and reserving judgement??
It seems pretty clear to me what happened here, but what I'm really obsessing over is a catalyst: what the hell happened around August 12th? They go from a seemingly blissful couple to attacking each other almost overnight.
Just spit-balling: I suspect Gabby may have had another suitor in her life. Brian probably found out by going through her personal media. He may have had access to her phone based on the suspicious final text messages.
To this point, however, the media has not done much digging into their recent past- yet.