how to sign in with Google

DO NOT JUST FOLLOW GOOGLE’s document, it is good as it should.
In short, using its demo and COPY ALL OF FRAMEWORKS to your project to avoid errors.

https://developers.google.com/identity/sign-in/ios/sdk/

http://www.theappguruz.com/blog/integrate-google-signin-sdk-in-ios-application-using-swift

https://github.com/simplyi/SignInWithGoogleAccountExampleInSwift