HF-GCS (High Frequency Global Communications System) uses a variety of frequencies, but if memory serves, the two primary frequencies that are always used are 8992 kHz (better for night propagation) and 11175 kHz (day propagation). If you're using one of the receivers listed at websdr.org you might try those for a start.

If you're in northern California and in need of a day trip, drive on Interstate 80 to the town of Dixon and head south on Highway 113 towards Rio Vista. Eventually you'll see a pair of very tall antenna towers. I'm pretty sure those are the "West Coast" HF-GCS transmitting towers, with the receiving antennas located further east near Lincoln.

(And if you're up for a great burger and shake, Dave's Giant Hamburger in nearby Fairfield will set you straight. (Be sure to ask for the hot sauce.))