The executed function of a Google Cloud Function can be a method on a custom type (Srv.Handle, here), under the condition that both the type and method be exported.

gcf-method-handler.go

package p

import (
	"fmt"
	"net/http"
)

var Srv = Server{}

type Server struct{}

func (*Server) Handle(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/plain")
	fmt.Fprint(w, "Hello, World!")
}