It's really easy to add a Google Cloud SQL database in GCP which then can be accessed by, for example, Firebase Cloud Functions. It's also very easy to mix/match parts of Firebase with GCP. For example, you can very easily deploy a server-side API on a plethora of GCP technologies (i.e. App Engine, Cloud Run, a dedicated Compute instance, etc.), deploy a front end with Firebase Hosting, use Firebase Auth for user authentication, then when you call your backend API you can verify the authentication using the Firebase Admin APIs.