I have a theory about this that I think may get deleted or the people with pitchforks will come to get me, but I'm going put it out there anyway. Since everyone here thinks Brain is a possible serial killer and murderer, let me counter that with this:
Gabby herself stated she was going through some things (I am not bringing up mental health issues, these were her words), and she also seemed distraught and inconsolable in that traffic stop where it was determined that she was the aggressor in the situation.
Put yourself in Brain's shoes please for this next part: Gabby has an accident, or she harms herself (a general hypothetical) or something other than Brian causes her death. They are alone, there are no witnesses. There is now a history of an altercation at the traffic stop and he is scared. Scared something happened and scared no one will believe him when he tells the truth (remember from Brian's POV). So he leaves her out there, not a great move, drives home, tries to figure out what to do next. He tells his parents what actually happened in his mind, she fell off a high rock and passed away, whatever he would have said. They call a lawyer due to this strange situation and are told to not talk.
Brian feels guilty and is distraught about her being gone and what he did by leaving her. He feels trapped with media camping out his house. He's scared, so he leaves. That's all I have. Anyone think this is possible? Not trying to give him a pass, just a different point of view, since no one, not me, not you, knows what happened.