A non-functioning temperature gage is usually caused by a corroded connector to the temperature sensor. The temp sensor is located on the LEFT side of the thermostat housing. It is a single wire connector. Insure this connector is firmly seated into the sensor and also check for corrosion or a broken wire. This is very common failure on Sportages.
The dash temp gauge can tell you if the motor is overheating, but it should be considered a general indicator only: it does not have sufficient resolution to tell you at (what) point the motor is overheating -
For example, in the '00 Sporty here, the temp. gauge consistently reads below 1/2 when running properly, if the motor is running hot / the electric fan is at full speed, only then does the dash temp. gauge read 1/2 ..
If you suspect the motor is overheating, suggest using (2) methods to confirm:
1) An InfraRed thermometer (IR Thermometer), w/ laser point sight: point the IR Thermometer at the thermostat housing (follow top radiator hose back to the thermostat housing block), that will give you a more accurate indicator on coolant temperature, about $20.00 or so US dollars at your local auto parts store or on Amazon: I picked up the [ OEM 25245 Infrared Thermometer ] here from AutoZone for about $25.00, and it is pretty accurate for a low cost unit, within +- 2 degrees (f) at higher temps..
2) Use a OBDII Scanner to monitor the temperature the Engine Coolant Temp. (ECT) sensor is reporting, see my sig. for the scanner I use,
connections, wiring: +1 on making sure the spade connector is tight and clean, and that the wiring is in good condition..
And always recommend cleaning up your battery, ground tray, and engine ground strap points to ensure good grounding through the vehicle,