There is more than one service providing API gateway functionalities. The following explanation helps sorting things out. The listing below focuses on API gateways and related functionalities of the mentioned services only.
The proxy service is a general purpose HTTP API gateway and manages HTTP-related tasks like transport encryption, authentication and others. Note that the proxy service routes incoming HTTP requests to other services.
All API endpoints that end users currently can use, are provided by the proxy. All other APIs and services need to be protected to not be accessible by end users and are only for service-to-service communication.
The frontend service acts among other things as the API gateway for HTTP up- and download requests. It is responsible for forwarding up- and download requests to the correct storage service, for example the storage-users service.
The gateway service provides a CS3 API gateway for gRPC requests to the CS3 backend.