Both conjunctive words are used to constrast two sentences in a row, but here is what I found over the Internet. 1) "그렇지만" is a contraction of "그렇(기는 하)지만"(it's just my own theory^^) 2) "그렇지만" is more often used in written than "하지만" 3) "하지만" can be conjugated with a sentence, but not "그렇지만". For example, 배가 고프다. 그렇지만 밥 생각이 없다.(O) 배가 고프다. 하지만 밥 생각이 없다.(O) = 배가 고프긴 하지만 밥 생각이 없다. = saya lapar tapi tak nak makan. However, in most cases both can be interchangeable. ^________^