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.

I've been playing with Pocketbase which uses SQLite:

https://pocketbase.io/

Supabase

Managed cockroachdb, along with some others like fly.io.

Supabase is awesome and open-source

Check out thin.dev https://thin.dev/ It uses SQL DDL statements literally as the building blocks for everything.

Supabase

SurrealDB is getting there soon. You should check it out!

Directus. Its really well built and polished. It supports postgres, mysql, etc.

If you're looking for something lighter, then use Pocketbase which uses SQLite.

Recommend Supabase.