I created a quiz game in Swift 4.2 that was NOT built using SpriteKit. All of the current resources that I can find about Game Center integration involve SpriteKit. Thus, I am trying to figure out if I need to make the game in SpriteKit in order to use Game Center? If not, can someone point me to current, relevant resource?
No, you don't need sprite kit, what you need is Game Kit framework, it's the framework that apple provide for you be able to have this connection. Try a look here first, for the apple documentation: Apple GameKit documentation And after looking at this look here for a easier tutorial: Implementing Game Center - Game Kit