Firebase and GCP have a noticeable overlap. Firebase primarily targets more mobile users. If you are a web developer, they do have a hosting platform, but thats more for static sites. A lot of the tooling focuses on mobile backends, auth, databases and analytics.
GCP is more of a fully fledged resource that can adopt to your various needs. Given the large number of products. You can still run mobile backends on GCP, but Firebase makes it easier for that.