Generally, GPSS really only adds turn anticipation. Meaning that it will start a turn before the waypoint, to most effectively change course, without overshooting the waypoint. How early it starts the turn is based upon ground speed and degrees of direction change. Need to be a tad cautious, as some waypoints are considered fly over waypoints and you are expected to fly over the waypoint before executing a course change. I suspect many forget this years after their IFR test.
If you fly off course and re-engage the AP, it will make a track to intercept the exact course it was last told to execute. If you want to go direct from the present, off course point to the next waypoint, just press DIR ENT ENT, assuming it works the same as the old 430 that I use. Never have used the touch stuff. If ATC puts you on a vector back to the original course after the deviation, some AP's will allow heading mode and have nav on standby to acquire. If not, go to heading mode and as the plane indicator reaches the magenta line, switch to nav mode for final intercept. I don't believe that I have ever been given a vector back to course; Always "resume direct" or given multiple vectors until told to resume direct.
This my understanding, but sure someone will correct me if I missed something.
Larry