Unless you are using a rain team, use gooey. The speed drop from gooey can really come in handy, and it may even force some opponents to switch out. Hydration is very good, but, when not on a rain team, it is pretty much useless. For an all-around team, go with gooey. Watch out for Contrary Malamar, though.
For the nature, I'd pick Modest. Gentle is good because it ups Goodra's already-superb Special Defense, but it lowers its not-as-great defense, leaving it vulnerable to physical attackers. So, Modest would probably be better, as Goodra's Sp.Atk is higher than its Attack stat.
It all really depends on how you choose to play. Hope I helped!