This just needed for Redis submission. Please use a different approach. Add String additionalFieldPrefix in the toJson-Method (GelfMessage.toJson(String additionalFieldPrefix)) and add a second, GelfMessage.toJson()-Method which calls 

    public String toJson() {
        return toJson("_");
    }

Using this way there is no need for adding configuration parameters in all frameworks and you can invoke toJson("") within the Redis sender.