What hub to purchase for a GT aggressor?
There are plenty of wheels that will fit that bike, and they may very well be less expensive than having the hub overhauled.

You want to find a 26 inch wheel with an 8/9 speed cassette compatible hub, with 32 spokes ( that's the 32H part )

I actually don't see any on Amazon.com that fit the bill. This is one of those times I would head into the local bike shop to make the wheel replacement process quick and easy.