Steve is correct. Try the auction system. Watch the auctions for a while and get a feel for what the GC are going for. Find one that you are interested in and bid. There seems to be the same GC week after week. Ihave won a pair of movie passes and krispy kreme GC so far. The KK GC went for around 320,000 points. ANd the points add up pretty quickly.
Good Luck!