I have a Goodman GMP100-3 furnace. Runs constantly. Air temperature coming out of ducts is luke warm. Burners all working fine. All vent pipes checked for leaks. Great air flow just no hot air.

Check this first >Low gas pressure . The unit is capable of 100,000 BTU input of heat and unless you are trying to heat the outdoors should be more than plenty for a average size house.

I had my gas pressure check by an HVAC Tech. Guess what, furnace designed to run at 3.5 lbs, input valve set at factory default of 1 lbs. Tech set pressure up and problem was solved.
Thanks for the tip.

Yep low pressure is the cause of many problems. Glad you got it fixed.