The casting of a gate spell has two effects: first, it causes an ultra-dimensional connection between the plane of existence the cleric is on and that plane on which dwells a specific being of great power, the result enabling the being to merely step through the gate, or portal, from its plane to that of the cleric; second, the utterance of the spell attracts the attention of the dweller on the other plane. When casting the spell, the cleric must name the demon, devil, demi-god, god, or similar being he or she desires to make use of the gate and come to the cleric’s aid. There is a 100% certainty that something will step through the gate. The actions of the being which comes through will depend on many factors, including the alignment of the cleric, the nature of those in company with him or her, and who or what opposes or threatens the cleric. Your Dungeon Master will have a sure method of dealing with the variables of the situation. The being gated in will either return immediately (very unlikely) or remain to take action.