The status light blinks continuously which the panel states is either incorrect twinning or incorrect primary polarity. It suggests that I check "reverse transformer," "secondary leads," and "reverse primary wires." My diagram does not label these parts.

Is there a part that I might replace or a wire I might correct?
m/n GMT045-3 (on furnace box)

First question? What triggered this? Did you change any part of the system of change the electric panel etc?

A gentleman changed out the circuit panel a couple months ago. The heater was working fine a couple days ago. Then it got hot so I turned on the A/C. Now it is cold again and the heater kicks, but does not come on. It starts, but does not go. Then the light just blinks continuously.

You could try to reverse the low voltage transfromer wires to see if that cures the problems.

Would you be able to tell me what the tranformer looks like and maybe even the color of wires to be switched? Would I switch these wires on the transformer or the circuit panel?
The connection that is marked "TWIN" on my circuit panel is empty and I have no lose/disconnected wires.

Look at page 14. Image 1. Drawing part number 301. On the link below. This unit looks like the transformer has push on connectors. Just swap the wires to the opposite connections to reverse the polarity of the 24 volt transformer.

http://site.famousparts.com/goodman/PDF/rp-299.pdf

LEAVE the twin circuit alone. It is only used when you have two furnaces tied together in the same location.

Naturally turn the power off to the furnace while changing the connections and make sure or mark the wires if you cannot remember where they came from to start with. But with only two wires it is hard to make a mistake. When done start the furnace up again and see if it works.

Thanks. My wife just told me that she had unplugged and plugged back in the unit in an attempt to reset it. She plugged it in upside down. It works again.

Glad you got it going