I have this multi layered application entirely hosted on GCP. At the moment, we only have the back-end part. Front-end and API are to be developed. For the front-end, the decision has been made - it will be a React.js app hosted on Firebase Hosting and the authentication method will be Email/password and users will be provisioned manually through the Firebase Hosting UI.
As we'd like to have a re-usable middle layer (API) we're in a process of making a decision what type of a solution to be used for our middle layer. The main request here is only logged in users to be able to call the API endpoints. Eventually, there will be also a native/mobile application which will have to also be able to make authenticated requests to the API.
My question here is, what type of GCP service is advised to pick here? I want it to be light, scalable and price optimized. Preferred programming language would be C# but Node.js would be also acceptable.