The Judgement Gate was put in place by the gods in order to test the strength of love between couple from mixed worlds. the perfect example of this is the relationship between Belldandy and Keiichi.
The couple must pass through the gate without any doubt or distrust in their hearts. If their love is not pure for these reasons then they will not come out the other side together and be forced to spend their lifetimes apart.
The Judgement Gate is introduced to the audience for the first time during the course of the Movie. Celestine believes that this gate has been put in place in order to keep disharmony between the worlds and the suffering that is brought forth by this gate is something that he can not accept. Morgan a fairy princess had once gone through this gate with a person that she had loved but she was forced to part with him forever. She blames the gate and the Gods for this heart break but has never really questioned herself in the matter.According to the Gods, this gate was set, not a a means to just test the love, but to make sure that the setting of harmony between to worlds was to be built on the foundations of true love. Those that past through the gate and emerge on the otherside together will see there love embodied in utopian setting.