GoogleService-info plist file is one of a file that you can download when you use third party service such as Firebase. A lot of times, we might forget to download or we could have lost the file. There are a lot of ways to find GoogleService-info.plist but this is what I am doing on my side.
Make sure you visit Firebase and click Go to console on the top right button and you’ll be brought to a page where all your apps that you have previously linked to will be shown here. Click on the app that you would like to download your GoogleService-info.plist
You’ll be brought to different page that shows you the statistics of your apps. Click on the settings icon found on the left and click Project Settings.
Here you will see more details about your apps and now you’ll be able to download your GoogleService-info.plist and import it into your project. In the event of you losing your file again, you may repeat the same steps to get your file.