Update: apparently it is fine to map to GSPs. I still think that the info below may be helpful so I'm leaving the answer up, but perhaps I have misunderstood your question.
Original response:
You shouldn't be mapping to or requesting gsps at all. They're used to generate views, but are not viewable without rendering.
Instead, go to url like www.xyz.com/customer/list
and map that like
"/customer/list" (controller: "customer") {
action = "list"
}
Or even better, you don't need a custom mapping for each endpoint. A default like this will work:
"/$controller/$action?/$id?" { }
Your CustomerController will render the list.gsp
in the list
action.