Multiplayer game with Game Center

Hi. For about 5 months ago, I started learning IOS Development(Swift) in order to create a game. I am now done with all my app and game´s features, and the only thing left to do is integrate game center, and its Multiplayer options. My game will be a turn based game

I have searched for tutorials on this matter, but I haven´t found anything. I do find tutorials on how to integrate leaderboards and achievements, but not on multiplayer option. I have tried reading apples documents on the matter, but it has not helped me much.

Is there anyone who have dealt with this feature of Game Center and want to help me?

If anyone has any experience with game center, may you please answer these questions:

1. As said, my game and its logic is completed. The only thing left is game center. How much of my app will you say is completed? 80 %?
2. If a guy, who knows game center, would get a task to create a simple turn based game, a game where the user taps a button to get one point and with it sends the turn to the next player.(Great game, i know) How long would it take for him to create it?

The reason I am asking these questions is because i may ultimately resort to pay a professional to integrate game center in my app, and I would like to know how much work there is to integrate it. Say, he wanted a percentage cut of my app revenue, how much would be fair to give him?

Help is much appreciated!!