The national parks were packed with tourists because it was during Covid. Gabby could have easily asked another couple camping nearby for help if they had an argument and she wanted to end the relationship.Does Uber go out to the desert?
Women Gabby's age would never hitchhike, I don't see her doing that unless Brian purposely stranded her without the van.
I believe Brian's written confession about Gabby falling and getting seriously hurt. They had videos of climbing and hiking. Not everybody who goes into national parks likes to hike the steep inclines.
JMO