Android : Manually parse part of a response when using Retrofit

I'm working with a REST API that returns a JSON document that starts as follows and includes a "collection" of items with string IDs like "ABC". Note the "routes" field, which contains a series of fields called "ABC", "ABD", "ABE" etc, however routes is not represented as an array in the json, so all these

{
"status":true,
"page":1,
"per_page":500,
"total_count":1234,
"total_pages":8,
"total_on_page":500,
"routes":{
    "ABC":[
    {
        "value":22313,
        <......>

I'm using Retrofit and the problem is the routes field is not an array (despite the fact conceptually it certainly is) and Retrofit/Gson require me to create a model object for routes with field vars abc, abd, etc - this is not practical as the data changes. For various reasons changing the server API is hard, so I'm looking to work around this on the Android client.

I figure these are options:

• 
  

  Intercept the JSON document before it reaches Gson and tweak the document, possibly with a customised Gson parser, or by intercepting the HTTP response.

  
  


• 
  

  Bypass the JSON parsing, and acquire the JSON document from Retrofit (I've yet to figure out how to do this, or if it's possible)

  
  


• 
  

  Use some feature of Retrofit I'm unaware of to map field names to a collection.

  
  

I'd appreciate help, especially if there's a quick and easy way to resolve this.