After defeating the Elite Four and becoming the newest Kanto Region Pokemon Champion, you can battle all of the Gym Leaders in a rematch on a daily basis back in their given Pokemon Gyms. Each Gym Leader will have a team far different than the one you faced originally, usually being much stronger than before with additional Pokemon too. On top of this, you will earn Experience and Money every day you defeat them.
When is comes to Koga, the Fuchsia City Gym Leader, his team hasn't changed all that much. In your first battle with him, Koga will use four different Poison Type Pokemon that are all at higher than Level 40. When faced in a rematch, Koga will have five Pokemon, four of which will be the same Pokemon Species as in your first battle with him (except at higher Levels). To be exact, all of his Pokemon this time around will be at Level 56, with the exception of his Muk that will be at Level 57. In all, Koga will battle with Tentacruel, Golbat, Weezing, Venomoth and Muk this time around.
Just like your first match against him, Koga's Pokemon Team will continue to be weak against Psychic Type Pokemon and Ground Type Pokemon. Psychic Types do best against Koga, causing major damage while not taking much damage in return. Ground Types should also be used in your rematch against Koga since their Ground Type Attacks can cause serious damage as well, just make sure to watch out for Tentacruel's Water Type Attacks. Both of these types of Pokemon should do for this battle, preferably at Level 55 or greater in order to match up well.