That's what NMEA-0183 (and latterly, NMEA-2000) is supposed to cover: these devices can talk to each other.
Although why an autopilot needs to "talk" to more than a fluxgate compass is beyond me, as I believe linking a pilot to a chartplotter and a bunch of GPS waypoints is risky business indeed (see my post in the "Obsolete Sextant" thread.
Actually, there is a good reason to do that aside from waypoints. My Raymarine S1G will use both heading and speed information from GPS to control the boat. Headings from GPS take precedence over those from the compass, when available - which may or may not be good I suppose, but certainly was always good in my case (fluxgate can fluctuate quite a bit in rought conditions and it's readings need a lot of dampening, GPS is a lot more stable).
Speed is very important, because when autopilot knows what speed the boat has, it can react differently to various conditions based on that (though on sailboats speed range is fairly narrow).
In addition, connecting autopilot to GPS makes it easier to set GPS's own fluxgate compass - just stay on a course, and press "sync" button. There are a few more good uses where autopilot - GPS linkage comes in handy.
Incidentally, I am quite pleased with S1G autopilot so far, which I can't say about the Garmin chartplotter, that died for no reason less than 6 months from purchase (and only after a few weeks of use).