I loved playing Syphon Filter back in the day. I don't know much about MGS though.
From what I've seen, Snake has victories against superhumans and stuff; obviously, he'll have a more impressive resume than Gabe. Gabe's games featured mostly real life limitations....
....so can't argue against Snake here basically. Gabe probably loses.
Jokes aside, if you don't take into account Snake's actions on Shadow Moses Island and all after that, it would be a bit closer, but I'd still go with Snake. He has taken down Metal Gears for most of his life.
__________________ Answering questions is questioning answers,
Yeah, definitely snake 7/10. You never know though, Snake could be tased to death or something. Also, he is not unkillable. For every uber human he takes down, he can be felled by some lowly soldier just as easily. I would say is at least a top tier agent giving him at least a decent chance of survival.