"world-reknowned balloonsman gabe newman conducts his maiden voyage through the towering steel skyline of new york, dragging behind him a life of regret. forever haunted by his experience in the cambodian 22nd balloon regiment and full metal jacket blanket party scene, gabe's grip on reality falters daily. gabe newman knew how to target women and chidren. by leading a little less to compensate for framerate"
Loads of mods, and probably a lot of patience as well. Also dementia might come into the picture when it comes to ideas for videos but that's just speculation.
GTA4 on the PC has a built-in system for saving data and then allowing you to edit together movies based on what you saved. This guy, though, like most people with GTA4 on the PC is using a ton of mods as well.