Not original work
My problem isn't that you used Garage Band, it's that none of these loops were made by you. You just took a bunch of the pre-made GB loops and threw them together. Garage band can be used to make some great songs, but that's not what you did here.
This doesn't sound bad, but it definitely shouldn't be rated above all these other songs that people actually made themselves. Anyone can make something as good as this by fooling around with GB loops; hell, most people do when they first try out the program.
Also, I don't know if its legal for you to licence this under the creative commons deed, which automatically happens by uploading to Newgrounds. Apple might have copyright on those loops.