Just to be clear,
GSP uses two simplex channels in a DUPLEX configuration. Like b7spectra (hey mike) said, they use 154.680 for base to cars and 154.800 for car to base. If you're close to a trooper on a road stop, monitor the 458.x and 765.x frequencies (listed above). Those are the Pak-RT (aka "pack-rat") style in-vehicle repeaters. Also, many troopers talk car to car on 154.680, since they can easily chat and listen when the Post dispatcher calls.
Look for GSP to be on neighborhood 800MHz trunked systems. Most troopers, across the state, have local access and/or talkgroups (e.g., Tift County, Atlanta). Wherever 800 MHz is used, expect to hear the troopers' in-vehicle repeaters on the 765.x frequency. That's a trend we all have monitored recently.
As a practical note, I usually travel with one radio set on 154.800, while in my home state. If you can clearly hear the trooper, that's your clue to slow down...
Like the GSP duplex system, 154.905 and 154.935 are two simplex channels used as a pair. If you are talking car to car, both are on 154.905. If talking base to base, use 154.935. Base to car traffic is on 154.905 and car to base is on 154.935. All CSQ, as are the 154.x frequencies for GSP.
The state car to base system isn't as heavy with traffic as it used to be, but then carrier pigeons aren't in as great of a demand either.....