GMC Jimmy runs cold

Earlier this winter I had a leak in my heater core. The leak was small(no noticeable loss of anti freeze) but I did get a anti-freeze smell and small amounts of vapor seeping into the cabin. I refilled the radiator every time I drove it for a couple of days(about 10 ounces gone after 20 min trip). On the third day the temp gage never rose just above cold. Bought some stop leak and put it in. Smell and vapor seemed to stop after a couple of trips. After stop leak did it’s thing, I ran the Jimmy in the driveway for about 45 minutes with the radiator cap off to see if I could burp out any air. Some air was removed and refilled radiator. Temp gage still reads just above cold on next days driving. I figured it was the thermostat(still had heat, though). Replaced that and refilled with anti-freeze. Temp still reads just above cold. Temp seems to read hotter when in park(but not at operating temp) on an idle rather than driving(temp actually goes down once driving).
I don’t know of any other things that may cause a cold temp other than low coolant or sticky thermostat. Any help would be appreciated. Thanks in advance!

Not sure if your vehicle has this but some GM's do. Look around the rad hose connection on top for a small bleeder valve. It may be air bound.
What year/engine?

1994 4.3 Vortec

Sorry no luck finding any info.

Mine ('98) was showing just over cold on guage, with semi-heat in the ducts. I swapped the thermostat, and the guage reads proper.

System was clean before, good core flow... Heat is still modestly hot. Air is almost all gone, I think.

Did you put a garden hose on the heater core?