10 years ago, I might have said, "Very good possibility he would just run!" But today, with all the bizarre variations in intimate partner crime/issues, I'm not so sure. He obviously stayed with the van (a big investment either for him or for both of them).
Lots of murderers try to choose the "not guilty-looking" option after some reflection. Let's go over the other options, though.
1. Gabby got lost in a NP or on a trail and BF failed to report it where it happened. (??? Why?)
2. Gabby got mad, made a definitive statement about leaving BF, and stalked off into...somewhere. Where? Trail head? Camp ground? Roadside? Hotel?
3. Gabby got mad, went to cool off, never returned (and BF failed to report it where it happened).
4. Gabby and BF were hiking, she fell or otherwise became incapacitated (and BF failed to report it when it happened).
5. Gabby told BF she would hitchhike home or take a bus home, after breaking up with him during the trip. BF doesn't call her family to check on her (is angry at her enough that he wants nothing more to do with her or her family?) and has not been heard of since. BF doesn't find it necessary to let the family know this, even now.
6. BF knows exactly where she is and that she's dead and not coming home and figures he needs a lawyer at his side while talking to LE, which he will do, one way or another, eventually (will he make LE get a subpoena?) For example, maybe she ran into a bad actor, BF knows this, but realizes he will himself be a suspect, as he can't identify the stranger.
7. BF knows exactly where she is, because he killed or harmed her, and therefore has lawyered up.
He may feel he needs to be on his home turf to cope with any of these scenarios. It will be treated as missing persons case in her old hometown, I would imagine (surely she had some kind of permanent address somewhere - although if completely off the grid, maybe not).