gps sats all transmit on the same frequency, using spread spectrum techniques, and all use a different spreading code... the receiver uses multiple correlators (often loosely referred to as channels) to be able to receive the different satellites simultaneously. the data rate from the sats is actually quite slow, hence it can take a long time to get a fix when the receiver's not been on for quite a long time in order for it to acquire the full ephemeris data (which is the satellite's deviation from expected position).

there are actually TWO gps frequencies, the full mil spec receivers use both as it helps counter atmospheric disturbancies and jamming.