I don't think it's the "injection rates" it's reading, or trying to read or associate. I'm not finding any reference to that, and it should make no difference, functionally. The GPC must be able to relay the resistance and feedback voltage for each plug, though.

Did you replace #4 GP? Others? If not, it could still be an issue, or wasn't the exact issue from before. What was the reason for replacing the GPC? Have you tried reinstalling the old one?

I'm curious to know what the service manual has to say about this DTC (not curious enough to pay for a manual subscription for a truck I don't have or work on). A thorough web search shows the only folks talking/talked about P106C in regards to a GM truck is you and me. The only other reference to it is older foreign cars with tranny issues on foreign websites, and even they are vague with no positive solutions.