If you take the monorail from MK, it will make stops at the Contemporary, TTC, and Polynesian before arriving at the GF. That will likely take 15-20 minutes once you board the monorail, depending on whether or not there are any traffic holds on the line and how long you sit at each station. It's usually faster to take the boat launch, unless you see the boat just pulling away from the dock as you approach.
The resort monorail runs pretty regularly - I doubt I've ever waited more than 10 minutes for it at the MK station, but if you assume 20 minutes between trains, you'll be safe.