When conjugating these types of sentences to the future, it is possible to attach ~겠다 to the final verb. For example: 밥을 먹어야 하겠다 = I will have to eat (rice) The construction of “~아/어야 하겠다” is often shortened to ~아/어야겠다 in speech. For example: 밥을 먹어야겠다 When adding ~아/어야겠다 to the end of a sentence like this, the translation of “I will have to…” doesn’t perfectly reflect the purpose of this construction. When a speaker uses ~아/어야겠다, he/she has received some sort of information or signal that is causing him/her to say this sentence. For example, if you just looked at the time and realized you had to go, you could say: 나는 지금 가야겠다 It is hard to come up with a perfect translation for this in English. Some translations for the sentence above could be: I’d better get going now I guess I better go now I’ll have to go now http://www.howtostudykorean.com/unit-2-lower-intermediate-korean-grammar/unit-2-lessons-42-50/lesson-46/

(1) 밤에 놀러가야 겠네. (2) 밤에 놀러가야 되겠네. It's very similar meanings. There's almost nothing the difference. In my opinion, if you say (2), there're some reasons before that sentence. Because It includes the meaning "therefore" or "result". ~되다. But, you can say both of them.