I think you only need the GISC if you're going to have the serial cable plugged into the PC all the time...because if you don't have the GISC it will cause your PH to not measure correctly.
So if you don't have a GISC, just don't leave it plugged into your PC all the time and you shouldn't have any issues (although you will have to plug it in occasionally to do the initial programming, grab readings from the ACjr, update programming...)