Even though the Game Center App has been removed it is still present in your iPhone’s settings.
Scroll down until you see the section of apps with Music at the top:
Now open Game Center:
You may sometimes have trouble with Game Center not syncing up correctly. 95% of users report a fix when they sign out and sign back in. To sign out tap your “Apple ID” when in the Game Center section of settings:
Select Sign Out then sign back in. When you relaunch the app it should re-sync up with our app and show your current games. Note that we must block games between major version updates, so app version 2.X cannot play games with app version 3.X