What exactly happens with a gmt 800 can probably only be told by an engineer who developed the car.
Normally, all cars have volatile memory in which, for example, adaptation values are stored, for example transmission shift points depending on your driving style. and there is a fixed memory that stores things that should still be there even after the battery is disconnected, such as errors, the immobilizer data and so on.
And at least the programmer of the ECM or other Controller decide themself if an error will disappear when the cause disappears, when the battery was disconnected or can only be activ deleted. That is the reason why in some cases the engine control lamp is on and disappears itself, or that Airbag error which was caused by low voltage is only resetable bei computer.
And besides. It doesn't matter whether you remove the positive or negative terminal of the battery. The reason why people always say that you should remove the negative terminal first, is just for safety reasons, because if you try to remove the positive terminal first and get to the engine or body with a wrench, there may be a short circuit. That cannot happen with the negative pol. If the negative pol is removed, there can no longer be a short circuit on the positive pole. Electrically, it doesn't matter whether the positive or negative pool is disconnected first.
If you want to reset the memory, you should even remove both poles and hold the two cables together, because then the capacitors also empty, which means that residual voltage disappears from the system.