Compilation of the RT podcast discussing Gavin's 'Run Down a Mountain' Theory.
I couldn't find any video of someone jumping out of a plane and running down a mountain in GTAV, the closest thing I found is this:
https://www.youtube.com/watch?v=6OOd-Fq7Wuc
There's also this real-life skydiving video that is a partly related to Gavin's theory:
https://www.youtube.com/watch?feature=player_detailpage&v=AiGdqPfztsc#t=56s