Gson - JsonObject with null values - json

Question

I am a little confused about how Gson is parsing Strings to JSON.
At the very start, I initialize gson like this
val gson = Gson().newBuilder().serializeNulls().disableHtmlEscaping().create()
Next, I'm converting my map to a String:
val pushJson = gson.toJson(data) // data is of type Map<String,Any>
That gives the following output:
{
"name": null,
"uuid": "5a8e8202-6654-44d9-a452-310773da78c1",
"paymentCurrency": "EU"
}
At this point, the JSON string has null values. But in the following step:
val jsonObject = JsonParser.parseString(pushJson).asJsonObject
it hasn't!
{
"uuid": "5a8e8202-6654-44d9-a452-310773da78c1",
"paymentCurrency": "EU"
}
Nulls are omitted. How to get all null values in JsonObject like in JSON string:
{
"string-key": null,
"other-key": null
}
#Edit
Added some json's to help understand the issue.

Answer 1

After discussing with the OP it came out that the JSON object was then serialised by Retrofit to allow for an API call, using the following code:
return Retrofit.Builder()
.baseUrl("api/url")
.client(httpClient.build())
.addConverterFactory(GsonConverterFactory.create())
.build()
.create(ApiInterface::class.java)
The issue here lays in the GsonConverterFactory: since no Gson object is passed to the create method, a new default Gson instance gets created under the hood, and by default it doesn't serialise null values.
The issue can be easily solved by passing the appropriate instance to the factory:
val gson = GsonBuilder().serializeNulls().create() // plus any other custom configuration
....
fun createRetrofit() = Retrofit.Builder()
.baseUrl("api/url")
.client(httpClient.build())
.addConverterFactory(GsonConverterFactory.create(gson)) // use the configured Gson instance
.build()
.create(ApiInterface::class.java)