We want to build a Javascript/HTML gui for our gRPC-microservices. Since gRPC is not supported on the browser side, we thought of using web-sockets to connect to a node.js serve
Unfortunately, there isn't any good answer for you yet.
Supporting streaming RPCs from the browser fully requires HTTP2 trailers to be supported by the browsers, and at the time of the writing of this answer, they aren't.
See this issue for the discussion on the topic.
Otherwise, yes, you'd require a full translation system between WebSockets and gRPC. Maybe getting inspiration from grpc-gateway could be the start of such a project, but that's still a very long shot.