From the GF you would have to make almost a complete loop on the monorail (MK, Contemporary, TTC, Poly). It is only a 10-15 minute walk from the GF to the Poly and is a very nice walk early in the morning. But if you would prefer the monorail it is probably about 15-20 minutes from the GF to the Poly. If you left the GF at 7:30 you should have no problem making the 8:05 ADR.
Unless you have mobility issues, you should definitely walk instead of monorail. In the morning, the monorail will be full of people going to the parks and you will also be susceptible to being delayed due to the monorail stopping for traffic or loading issues or whatever. Time to walk depends on what building you are in at the GF, but it shouldn't take you more than 10-15 minutes.
It is a really nice walk, and much shorter than it looks. We always walk between GF and the Polynesian.