From Nookipedia, the Animal Crossing wiki
Gulliver (ジョニー, Jonī, Johnny) is a gull in the Animal Crossing series. His only function is to reward the player with special furniture, usually artifacts from around the world that cannot be obtained anywhere else in the game, which suggests he is an explorer. The name Gulliver is a reference to Gulliver's Travels, a book by Jonathan Swift about a man who travels to the far corners of the earth, before being shipwrecked and washing up on the shore of a previously undiscovered land.
In Dōbutsu no Mori, Gulliver has the same role appearance from Animal Crossing, but instead of giving a rare furniture item, he gives only a random one. When the player talks to him, he may speak English, despite the game being all in Japanese.
Unlike in later games, Gulliver appears as a sailor in Animal Crossing. Gulliver can sometimes be found lying on the beach in Animal Crossing. He must be first woken up. Players do this by talking to him multiple times. He then tells the player an unbelievable and dramatic story about his adventures at sea, sometimes involving a giant squid which is assumed to be a jellyfish. Gulliver then gives a rare piece of furniture to the player, thanking them for helping him.
It is also possible for the player to see Gulliver floating in the ocean during a trip to the Island. When the player returns, Gulliver will likely be located somewhere along the beaches.
In Animal Crossing: Wild World, Gulliver is no longer a sailor, but an astronaut that infrequently flies over the player's town in a typical UFO styled spacecraft, called Porpoise, a possible reference to the ship that Captain Olimar uses in the Pikmin series, called the Dolphin. The player can shoot him with a slingshot, causing him to crash somewhere in the town. The screen cuts to the scene of the crash, which is always situated on top of one of the signposts that are scattered about the town. After being woken up, Gulliver does a quick survey of his craft, revealing that five parts are missing. These parts, which are dotted around town in the form of cogs called a UFO part, must be recovered by the player in exchange for a rare item. Gulliver and his spaceship then disappear the next time the game is played.
It should be noted that he does say that he hopes the player will see him off the next morning, even though he goes the next time the player plays again.
In Animal Crossing: City Folk, he retains the same appearance as before in Animal Crossing: Wild World. This time, without the dual screen as with the Nintendo DS, his spacecraft can be seen flying about the same height as a floating balloon. The player can shoot at him with a slingshot, causing his ship to malfunction and appear the next day on the beach. During this time, a piece of his spacecraft will land somewhere in the town. He takes a while to regain consciousness, usually about five attempts of talking to him. When he wakes up, he will survey his surroundings and make unusual remarks about the player's appearance. He will ask the player for help, to find his spaceship part, which appears as a small, cog-like item on the ground called a 'UFO part' (City Folk) or 'spaceship part' (Let's Go To The City). When given the part for his ship, he will give the player one of his rare items.
Gulliver appears in Animal Crossing: New Leaf as he does in Animal Crossing, washing up on the shore of the beach wearing his sailor outfit. To wake him up, the player may either use the megaphone and say his name into the 3DS's microphone, or try to talk to Gulliver a number of times. Once awoken, he will ask the player where he was heading before he washed up on shore, providing the player with key information about his destination, a country in the real world, such as the United States. When answered, Gulliver may send a letter the next day containing a present from the country he is visiting.
 Other appearances
 Gulliver's Rare Items
 Names in other languages