From the MK to the GF you can take the monorail or a boat (Gold Flag Line??). I've never timed either of these options, but I think if it was me I'd be walking out the MK exit at 1:00PM. You'd probably be early, but that's the way I like it, so I don't have to stress.
If you take the Monorail make sure you board the Resort Mono, not the express Mono. The resort mono will stop at the Comtemporary, the Ticket and Transportation Center, and the Poly before the Grand.
The boat will go directly to the Grand but you could wait awhile for it if it is just pulling away as you walk up, probably not more than 20 mins though.
I wouldn't drive as you would still have to take the boat or mono to the parking area and then drive over and park again. I think that would take longer.
Shelly