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.
^________^