Do NOT use Dexron VI in a manual transmission or transfer case. The "Manual Transmission Fluid" is Dexron III.It is certainly in the realm of possibility. Or perhaps the GM fluid is exactly in the range Tremec engineers specified. That’s what makes the fact that Tremec has made a change to the synchronizer interesting as well. If they thought the solution was fluid it would be a lot easier to just change the spec.
Also, people say Dexron VI is the same as the manual transmission fluid specified in the owner’s manual, p/n 88861800. It would be interesting to know if they have the same friction modifiers or not.