You may simply plant your apple seeds and they will likely produce many apple trees for you. However, the fruit will not be true. This means that you will not be able to raise Gala apples off of seedlings from Gala apples. The only way to reproduce the apple and assure yourself the same "type" of apple is through grafting. Grafting a small piece of wood (known as scionwood) from a Gala tree to a known apple rootstock guarantees that all wood above the graft will yield the same fruit as the parent tree (Gala). Anything that grows from below the graft will yield something entirely different and most likely be completely useless. In this way, thousands of apple varieties have been kept around for hundreds of years.
By the way most seedlings will start to produce in 10 years or less. If you use the right rootstock, a small piece (2 inches) of scionwood can begin to produce fruit in 3 years.