So how far off an airway is an aircraft when they are following a GPS flight
plan with every intersection and VOR?

If you miss entering one of the bends in the airway it could be 20 or
30 miles off. The point is that you are now back to the 1970's,
grabbing your enroute chart and trying to figure out all the airway
bends when you could have a 1990's handheld GPS (King Skymap IIIC) and
it would do it all for you.

-Robert