Ed,
I would expect this if the referred GPIO pins/port is configured for input by mistake - in this case you would only be able to read back what is being fed to the pins. Keep in mind that CCS performs standard memory accesses to these memory mapped registers, but the results and functionality are completely dependent on the peripheral/device configuration.
In this case, perhaps you will have some better luck at the C2000 forum itself - they are more knowledgeable about the peripherals themselves.
Regards,
Rafael