My autopilot can be coupled to the GPS. It gets it's left and right
from the GPS's OBS (sort of a VOR head for a GPS). In calm air it is
PERFECT. The track lines are straight as they can be. Outflies any VOR,
NDB coupling or anything else. GPS is accurate to a few feet right and
left. In over 2000 hours of flying behind my GPS, it has never failed
on me for any length of time. (A very few lockups and lost signals due
to military jamming, but VERY FEW and never for more than 5 minutes).
GPS coupled autopilot is the way to go. Nothing else even comes close.
Couple your autopilot to the GPS if you can couple to anything. It's
phenmenal.