(Originally answered for OkHttp version 2.5.0).
Replace
String json = response.body().toString();
with
String json = response.body().string();
response.body returns a ResponseBody object, which has its own string method: see the source here.
response.body
ResponseBody
string
kotlin user try this code 100% tested
val result: String = Gson().toJson(response.body()!!.string())