You would be riding between 30 and 45 miles on a trail with a ruling gradient of less than 2%. How many rest stops do you need? If you look up the GAP's official site you will find a map showing the locations of facilities:

http://gaptrail.org/tmi/maps.cfm

Click on each section to see the location of bathrooms, stores, etc.

I doubt they will truck porta-potties to strategic locations on each day's route. The GAP is very user-friendly, with ample services available in most places.