Convert json to a serialized object

The problem I have is that I receive a data model of a service in json format. This model can not be modified on the server side. The problem is that some fields of that model are not consistent.

I am using a data model in kotlin in the following way:

data class ResumenModel(
        @SerializedName("fGestiones") val fGestiones: FechaGestionModel? = null,
        @SerializedName("gestiones") val gestiones: List<GestionModel>? = null,
)

 class GestionModel (
        @SerializedName("ID") val ID: String? = "",
        @SerializedName("noCtes") val noCtes: String? = "",
        @SerializedName("detalles") val detalles: List<DetalleModel>? = null

)

The problem is with gestiones in ResumenModel . In some requests, gestiones is an array of GestionModel and in other requests, the service responds with a GestionModel object. So when using Gson to create the object, you can not do it since you can not cast the object to arrayList or vice versa.

I know it is working well because when I ignore gestiones in ResumenModel , fGestiones if it is filled correctly (and it is another data model but simple with primitive variables)