I'm not convinced that the Galatian 3 quote shows a strong anti-slavery sentiment - I interpret it as equating slave/free with male/female i.e. a natural state that people are in (similarly, the Jew/Greek dichotomy).

Gregory of Nyssa shows a strong anti-slavery view which is commendable, but I feel that he was ignored by a lot of Christians (it's the first time that I've heard of his name, but then I'm an atheist).