Check engine light on oreily said low temp . change tstat?

possible and an accredited radiator shop will be able to do a flow test to check that
unfortunately , it indicates a gasket or cracked problem so have a compression test done to eliminate that
if that test proves good then have the thermostat removed and checked/replaced if the engine runs a viscous fan hub then check that as well
viscous fan hub failure is most noticeable when stopped at lights ( temp gauge goes up but comes back down when moving along the road) temp rises in slow moving traffic , using high engine rpm as in up hill or any condition where the air is not moving through the radiator unless the vehicle has speed
if you have electric fans then the problem may be in the coolant SENSOR unit that sends the temp to the ECM to switch on the fans

I think you are referring to a different indicator light. A glow plug is a Diesel engine part and I do not think an American version has a Diesel. There is a "check engine light" which is an indicator of trouble and will display as you described.

Go to a Advance , Autozone, or Oreilys autoparts store. You can ask for a free scan to diagnose your problems. The coolant sensor can relate to a cold start problem. Some models use 2 temperature sensors. One operates the dash gauge and the other operates engine controls.

You will need to know if you replaced the sensor to the dash gauge instead of the engine control sensor. Both use heat to set engine controls.

my used 4 cyl jeep radiator would only drain 1/2 of the fluid. jumped up and down on bumper and this dislodged the sludge in radiator and/or engine. flushed twice, then now notice low temp reading on gauge (out of specs) due to a low temp thermostat that someone installed due to previous over heating (sludge). Replace with factor spec thermostat and everything is fine.

the engine light is telling you something has gone wrong and you should stop and check it out. The low speed indicates that the CPU is in low power mode to protect the engine. The low temp reading is suggesting no coolant in the engine. Check oil and coolant level and have a fault code reading done before you cause more damage to the power train

Go to an Autozone, Advance, or Oreilys parts store for a free scan. There is a good chance that the trouble Code light will identify your problem.

Defective thermostat probaly staying open, Or a blockage in the cooling system , Flush out the whole ststem, End don,t have the front of the vehicle raise up,

My guess would be the temp sensor has failed or is unplugged.
What is the trouble code stored in the computer ?

Is it cold where you're at?
Have you taken the wire for the gauge off the sender and grounded the wire? The gauge should move to hot if the gauge and circuit is good. Also the connection needs to be clean.
Did you use some kind of sealant on the gauge's sender? Too much can interfere with the ground. The sender needs to make a ground to the motor through its threads. The resistance to ground changes and that is what moves the gauge.
If the Tstat was stuck open and the weather is cool that could also be a reason why the temp is low.
What temp Thermostat did you get?

Find the radiator hose that is attached to the front of the engine, under the housing the hose is attached to the tstat is located. Make sure you get the right temp tstat and a new gasket for the housing. Drain the radiator fully before you start the job or U will have antifreeze all over the place.