Dark post ahead, read with caution.
What are the non-"Brian murdered her" possibilities?
1) Third party murdered her - this is possibly the most sinister outcome of all. Two other people have gone missing in this area.
2) Weather/heat/lack of water
3) Wildlife (pack of wolves, a bear, mountain lion)
Brian's behavior after the fact is separate, but if any of the above were actually true, I think could be explained no matter how far fetched. Co-dependent, fight happens, they stop communication, he gets the gut feeling something wrong has happened, takes off into the swamps knowing she's missing so he also wants to be missing. Far fetched indeed.
4) Ok, this is hard to write. Suicide. Two possibilities I see here: Perhaps Gabby was the abuser and threatened suicide, had done so in the past, and it has been a habit that has been building and she executed it either purposefully or mistakenly, as a way to inflict abuse on Brian. Other scenario: After an extreme argument, maybe a breakup, she left feeling so awful that she felt this was the only way out.
With that said, it can be difficult to make any of these alternative situations/scenarios fit in with knowing what what we know about Brian and his behavior after the fact. I am sure the autopsy will resolve what happened, which will make this post look indecent.