There are "Pokémon Event" and "Pokémon Festival" Ribbons which have been distributed at Pokémon events and festivals. The description of these Ribbons may be "A commemorative Ribbon obtained at a mystery location." if the player has yet to obtain a Pokémon with one of these Ribbons themselves from an event - only obtaining a Pokémon with one from a trade.
Gift Ribbons available after Generation III normally prohibit Pokémon with them from being traded in the Global Trade Station.
So a gift ribbon doesn't do anything, just special to an event Pokemon, since you can't get it in the game.
More info on ribbons/source