Ganesh is a Hindu God, here is the story of how he came to be and why he is know as the gate keeper.

Lord Shiva, the Hindu God of death and destruction was away at a war. His wife Pavarti, who was a goddess in her own right, wished to bathe. Having no-one to guard the door to her apartments she conceived of the idea of giving birth to a son who could provide this service for her. Thus Ganesh was created, since goddesses have the power to do such things.

Pavarti gave Ganesh instructions that he was to stand guard at the entrance of her apartments and that he was to admit no-one until she gave word that it was OK to do so. Ganesh took up his post and undertook his assignment enthusiastically.

In due course the Lord Shiva returned from his war and went to enter. Ganesh, according to his instructions, forbade him entry. Shiva was enraged by Ganesh’s impudence and drew his sword and cut off Ganesh's head.

Pavarti emerged to find Ganesh decapitated and flew into a rage. Even though he was immensely powerful Shiva was upset with Pavarti’s rage. He swore to make amends by taking the head of the first living thing he found to replace Ganesh’s head. The first animal he came across was an elephant. Accordingly he took the head from the elephant an placed it on Ganesh’s body.

Thus Ganesh is depicted with an elephant’s head. Because of his role as protector of Parvarti's door he is associated, in Hindu culture, with protecting entrances.