Gson Parse Json with array with different object types

How can I parse this JSON using Gson? I have an array with multiple object types and I don't know what kind of object I need to create to save this structure. I cannot change the json message (I don't control the server).

The only class that function (sort of) was this

public class Response {
    private List<Object> tr;
    private int results;

    (...)

}

JSON Message (Note the array with multiple object types.)

{
   "tr":
   [
       {
           "a":
           {
               "userId": "112"
           }
       },
       {
           "b":
           {
               "userId": "123",
               "address":"street dummy" 
           }
       },
       {
           "a":
           {
               "userId": "154"
           }
       }
   ],
"results":3
}