GzipResponseWriter字段声明

Question

I am looking at this type

type GzipResponseWriter struct {
    gw *gzip.Writer
    http.ResponseWriter
}

And functions that will implement it

func (w GzipResponseWriter) Write(b []byte) (int, error) {
    if _, ok := w.Header()["Content-Type"]; !ok {
        // If content type is not set, infer it from the uncompressed body.
        w.Header().Set("Content-Type", http.DetectContentType(b))
    }
    return w.gw.Write(b)
}

func (w GzipResponseWriter) Flush() {
    w.gw.Flush()
    if fw, ok := w.ResponseWriter.(http.Flusher); ok {
        fw.Flush()
    }
}

Does the http.ResponseWriter relate to the second field? Why not gw1 http.ResponseWriter?