The easiest way without your own car would be to take a bus to MK and then the resort monorail to GF after that take the bus to DTD from the GF. The buses do not run from resort to resort.
Hope this helps!
The GF is on Seven Seas Lagoon - the same lake that is in front of the MK.
If taking the monorail from the MK to the GF - the GF is the last stop before it returns to the MK while the GF is the first stop for the Gold flag motor launches from the MK.
Be ready though, it is a long bus ride from GF to DD. I'd go backwards and take the boat from POR to DD and then go to GF from there. When finished at GF, take the monorail to MK (1st stop) and then catch a bus back to POR from MK.
Be ready though, it is a long bus ride from GF to DD. I'd go backwards and take the boat from POR to DD and then go to GF from there. When finished at GF, take the monorail to MK (1st stop) and then catch a bus back to POR from MK.