Hubs compatible with 7 speed cassette
The GT Aggressor 2.0 2014 ships with a rear wheel which uses a freewheel, so if you want to go 9 speed, you need a rear wheel which has a Shimano-compatible cassette freehub as well as a 9 speed rear shifter which is Shimano compatible (and obviously a 9 speed cassette).

Buying a new rear wheel will be likely cheaper and more reliable than having the existing rear wheel rebuilt with a freehub. A 9 speed cassette freehub can use a 7 speed cassette with a spacer. 

However, I'd recommend against the whole idea -- by the time you've done this, you've spent likely half the cost of a cheap bike like the Aggressor 2.0 and with the low spec of the rest of the bike, you're not really going to see performance gains (especially for the amount of money you're spending). If you want 9 speed, sell the bike you have and buy something is 9 speed already.