Top 5, dead or alive.
[UPDATE]
Wait you mean fighting right now? I don't think he'd do very well. I think you could pit him against a few guys like Mike perry, Mickey gall, Niko Price, or Alex morono. Maybe he could beat guys like Robbie and Maia, who most agree are clearly past it and vulnerable. Carlos Condit is another guy who is ranked too highly atm, he'd be favored against Condit even at 40.
But I don't think he does well against any guy a near decade younger with more athleticism, speed, power, and a full array of skills with tons of footage of GSP. There's almost no one in the history of MMA that could legitimately fight their way to a title shot and win it at 40, especially not in one of the deepest divisions today.
It would be hard to crack the top 20 without serious planning and good matchups, and even then GSP's chin won't be able to take some exhausting competitive scrap. He needs matchups where he can overpower low output opponents.