class A {
  List<BaseClass> items;
}

A obj;
........
gson.toJson(obj);

class A {
    List<BaseClass> items;
}

class BaseClass {
    String field;
}

public class CustomSerializer implements JsonSerializer<A> {
    @Override
    public JsonElement serialize(A src, Type type, JsonSerializationContext context) {
        JsonObject items = new JsonObject();

        for (BaseClass item : src.items) {
            if (item.getClass().getSimpleName().equals("BaseClass")) {
                JsonObject currentItem = new JsonObject();
                currentItem.addProperty("field", item.field);
                items.add("item", currentItem);
            }
        }
        return items;
    }
}

GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(CustomSerializer.class, new CustomSerializer());
Gson gson = builder.create();

A obj;
........
gson.toJson(obj);