Firebase Team AMAView other answers to this thread
Oops! I have another one.
Question about Firebase and GCP.
How is Firebase different from GCP?
Are there some duplicate services between the two platforms?
When to choose Firebase over GCP?
The short answer is that, underneath the hood, Firebase IS a GCP product. When you create a Firebase project, Firebase creates a GCP project for you behind the scenes. So services like Cloud Firestore and Cloud Storage are powered by those same corresponding cloud products -- there really isn't a difference between them. And so if you wanted to go ahead and add in some other GCP services to that project, you can go ahead and do that.
That said, Firebase provides some additional easy-to-use client libraries to access these, along with access to other services that aren't really part of Google Cloud like Crashlytics, Remote Config, and GA.