The temperature gauge for the engine should be about in the middle of the range when the engine is warm. If it is indicating cooler, then replace the engine coolant thermostat.
Also there can be multiple problems. The temperature gauge for the engine can also be reading wrong. The cause of this is usually the "temperature sender". (NOT the engine coolant temperature "sensor" which is for the engine computer!) Then when you fix this, you might then see the temperature is too cool, then realize you also need a thermostat.
And there can be a third problem. The radiator may be clogged and the engine was overheating in the summer, so someone removed the thermostat to get it to cool better! For this new radiator and reinstall thermostat.
The engine thermostat should keep the water circulating hot. Even better is to use a factory service manual along with a thermometer and measure the temperature of the water. Should be what it says in the vehicle factory service manual set.
Anyway once you are sure the water is hot, then also the above about the water being able to circulate through the heater core can be a problem. (As they said above.)
And then there can be a problem with the little "door" in the HVAC system which switches from hot to cold (blend door). If you have an electronic HVAC system, do the following...
Try a "reboot"!
Remove a battery terminal for 10 minutes, then reconnect.
Turn ignition switch to on, but don't start.
Wait 1 minute.
DO NOT touch any buttons in the vehicle during this minute.
Then start vehicle and let idle for 5 minutes.
Drive above 45 mph as soon as possible.
This "reboots" all the computers in the vehicle including the A/C which then "resynchronizes" the actuator doors during the power-up.
This also reboots the engine computer and the idle and driving above 45 mph helps it to "regain" its operating memory.
If this does not work, you would need a GM Factory Service Manual set which would have complete troubleshooting instructions for this problem as well as complete part replacement procedures. Order from helminc.com or a GM dealer.
You would also need to know how to use a multimeter along with the instructions in the manual. How to use a multimeter...