No, you can still buy your badge online! If you buy it online now, then you'll just have to pick it up at Will Call when you get to the convention, rather than waiting in the humongous general admission line.
Just go to
www.gencon.com and click on the Registration links. You'll have to sign up with their website, but it's a pretty easy process.
GenCon actually changed the way their badge purchasing system worked this year. It used to be that by about this time at the end of July, you couldn't buy them online anymore. But now, it seems that you could probably buy it online on Wednesday, Aug 4, and still pick it up at Will Call the next morning.