Gates are obstacles in Braid. Each Gate requires a Key to open, although any Key will do, i.e. there are no specific Keys for specific Gates.
There are two exceptions to gates opening by a key. The first is two Monstar Gates that open when all the monsters are dead on the map, located in 2-3: Hunt! and 4-4: Hunt!. The other exception is in 1-1: Braid, which is removed from the map rather than given a key.