There's not much you can do with that stock bull barrel on that particular GAMO rifle, being as that it's covered in polymer/plastic material. That being said, you can probably have a tuner cut off the barrel -- retap the barrel housing -- fit on a threaded barrel and recrown it with a MuzzleBrake....however, that'll cost you a bunch. So I would go with either a tune, or a Gas-Ram from PyramydAir....both will end up costing you the same amount, unless you get a good deal somewhere. Good luck!
-Jarred