GFS GRIB data on the 1/2 degree resoulution Have been available from Saildocs for some months now. Newly introduved on zyGrib, if you ask for 7 days you will get 8. Presumably, they will amend the menu in time. The app from Weather 4D also has 8 day GFS GRIBs. See http://weather.mailasail.com/Franks-Weather/Grib-Files-Apps
The usefullness of the 8th day is rather doubtful just now. Clearly, there must be some skill or the US would not issue the forecasts. Since the last major enhancement of the GFS, there has been a noticeable improvement in the days 5 to 7. In time, no doubt day 8 will besome more useful than just now.
Watch this space.