Remember you should check in for your reservation 15 minutes early. So working backwards:
7:10 reservation time - 15 minutes early
6:55 Arrive at 1900 PF - walk from monorail to restaurant 5 minutes (if you don't get distracted)
6:50 Latest you should arrive at the GF by monorail.
Now, how to do that from Epcot? Allow 15 minutes to wait for the monorail to arrive (you may get lucky and one will come sooner, but...)
The monorail to the TTC will take about 15 minutes to load, transit and unload.
Allow 10 minutes to wait for the resort monorail.
Figure 10 minutes for the resort monorail to transit to the Poly, load/unload and then proceed to the GF - assuming no "delays due to traffic".
All that adds to 50 minutes, Subtract from 6:50 (above) desired GF arrival time and you should be walking up the monorail platform at Epcot at 6:00.
YMMV.