Check with VCDS and see if the number it shows at the ECU is similar to that shown on the gauge (which isn't labeled, but "normal" is in the ~190F range.)
The CTS has two separate sensors in it; one feeds the ECU, the other feeds the cluster display. If they are not reasonably identical change it.
If they ARE the same odds are your thermostat is stuck open partway. This is pretty-common on these cars as they age; change both the themostat and the housing pipe and O-ring (the pipe is cheap, it's plastic, and if it warps it will leak. In addition pay attention the the torque spec on those bolts -- it's sealed by an O-ring, NOT brute force, and you'll warp or crack the pipe if you crank on it beyond specs.) I strongly recommend popping for the OE thermostat even though it is quite a bit more expensive; the aftermarket ones sometimes are fine, sometimes not, and for the difference in money it's not worth having to do the job twice. You'll probably need some coolant (get the correct stuff; you CANNOT mix parts-store coolant with what is in there) as it's nearly impossible to get enough out of the block to prevent a fairly material mess when you remove the thermostat feed pipe to change the thermostat.