[ He could have stayed and watch in morbid curiosity, but it would be wiser to put as much distance as he can between himself and Venom.
Whatever's happening to his buddy, it's not pretty. He can get into big trouble -- not to mention the others who can't afford to make self-copies.
Gabriel leaves several more copies of himself (and of those jerks back home he doesn't like -- he would've loved to see them die a horrible death) as he snaps himself onto the building's rooftop to catch his breath, and give himself time to think of what to do next... ]