The gargolyes are on old churches because the church represents a microcosm of the world. On the edges of the world are the places and things we don't understand and cannot integrate into our lives (at least not yet), which is why medieval monsters are composed of pieces of different beings put together. The outside of the church represents the Chaos, the Wild. If I understand it correctly, inside, you have the entranceway, representing the process of coming to faith: inside the building, but not in the sanctuary, and it is where the catechumens (not yet baptized) originally departed to after the lessons and before the Eucharist. The sanctuary represents heaven, and the alter, slightly raised, represents the most holy place.

The gargoyles are not to create fear, they are simply reminding people that as you enter the church you are going (symbolically) from chaos into the Kingdom of God, which is the process of the life of the Christian.