We have a the GSG-5 rifle. It is one of the most fun to shoot firearms we have, and the wifey and I both enjoy shooting it. From my experience and what I've read, it seems like most people have good luck with the Federal 550 bulk pack that you can get at Wally World for about $14 or so. Definitely try that stuff out on your new acquisition.
btw - is that latex, or rubber?