Buses start running to the MK between 6:15 and 6:30, but they don't run quite as often as the regular buses do. If you just miss a bus, you may have a longish wait for the next one.
So....for an 8am ADR at the GF, I would be out at the MK bus stop by 6:45. Then, the resort monorail will be running when you get to the MK, so hop on that and head to the GF. If the boats are running that's an even better option, but I can't say for sure what time they start first thing in the morning.