Deserializing an abstract class in Gson

I’d suggest adding a custom JsonDeserializer for Nodes:

Gson gson = new GsonBuilder()
    .registerTypeAdapter(Node.class, new NodeDeserializer())
    .create();

You will be able to access the JsonElement representing the node in the deserializer’s method, convert that to a JsonObject, and retrieve the field that specifies the type. You can then create an instance of the correct type of Node based on that.