Like Google Sign-In for Android, you will need a configuration file.
Go to the below link and create a configuration file be giving your ios-bundle identifier.
Here I will be explain with the help of CocoaPods for integrating the Google SignIn SDK with the app.
So first you need to install CocoaPods.
For that Open a Terminal…
and execute command.
sudo gem install cocoapods
Now it will ask for Password and Enter your Mac Password.
Once the installation is complete…Navigate to your project directory in the Terminal itself.
Then execute command…
This will create a PodFile in your project directory.
Now Open the PodFile
open -e Podfile
Now Enter these in your PodFile and click save…
xcodeproj 'PATH TO YOUR PROJECT' pod 'Google/SignIn'
Now come back to terminal and execute
CocoaPods installation set up for Google SignIn is now complete…Now we will go and integrate the SDK in the app..
You can find a Good Documentation here from Google Itself…
If things didn’t work out, you can download the Complete source code from below link.