Both 11 and 29 bit IDs can exist on the same CAN bus at the same time. Usually traffic on a high speed CAN interface will use 11 bit IDs while single wire can will use 29 bit IDs. But, you can put both on the same CAN bus. VW vehicles will do that.
GMLAN is not really the same across all years. The older style cars use what they call GLOBAL-A. This is pretty much standard across all the cars but they still use their own specific CAN traffic outside of the GMLAN spec. And, I think they did add to the specification over the years but more or less the GMLAN functionality is the same. Still, do note that the rest of CAN traffic can change from model to model and year to year. Just because a given car sends its gear in message 0x204 doesn’t mean that the next model year still does or that a different vehicle from the same “manufacturer” will. Not all buicks are alike, etc.
But, now they are coming out with GLOBAL-B vehicles. It is my understanding that this changes some things. But, it’s still kind of secret. I have two Global-A vehicles but no Global-B vehicles.