Nats need a solid number 3, a 1/2 ace. Gio is a current-day Ed Figueroa: wins enough in the regular season and then falls flat. Ross might become good, in 2019, but it's a guess. Roark slumped, but he's probably at least a 4/5.
Who?
Why do the Nats "need" a solid 1/2/3? Neither World Series team had a good number 3 pitcher. You could make the argument that Keuchel was pretty meh and the Dodgers didn't anything resembling a good number 2.
The Cubs had relatively decent starting pitching, but he Yankees certainly didn't.
Unless there is an injury, I don't see why they would need to make a move for a top of the rotation starter when they're in a division that, frankly, they should win fairly easily. Unless that deal is for someone who is going to be here long term, I'm not sure why you give up prospects now, when you can hold on to them and get what you need at the deadline (like a closer or a replacement for an injury).