Both the Follow Me car going into the grass AND the mismatch between the scenery yellow line, are probably a result of the scenery AFCAD not being accurate. For the Follow Me car, it's possible the AFCAD you are using might have misplaced nodes, orphaned nodes, missing links, un-connected parkings, etc. GSX needs the AFCAD to be clean without such errors, in order to work correctly.
The mismatch between the scenery yellow lines and the parking evaluation is also a result of the AFCAD not being accurate: on a default scenery, the yellow lines you see, and the AFCAD always match 100%, because they ARE generated by the AFCAD itself. Instead, on 3rd party sceneries that use custom yellow lines (not AFCAD-generated), the scenery developer should try to match the custom lines with the AFCAD as closely as possible.
The only information about a scenery GSX has, is from the AFCAD so, if this doesn't match 100% the scenery, you can't rely anymore on the visual scenery to be guided.
The GSX marshaller will always try to put the reference point of the airplane (the red cross when seeing from the Top view) in the center of the parking, as defined in the AFCAD. Which is the same position FSX would place your airplane if you would use the FSX "Go To Airport" menu. The parking evaluation is based on that position.
So, in order to get a good evaluation, just trust the Marshaller.
Also, if a 3rd party has a custom docking system, GSX doesn't know anything about it, except its eventual presence, which can be set or unset using the scenery customization editor. But notifying GSX about the existence of a docking system on a 3rd party airport using the customization, will only be used to stop GSX to generate a Marshaller on that parking, but still GSX will expect you will park your airplane with its center in the parking center as defined in the AFCAD. If a 3rd party scenery uses a docking system that is set to have your airplane parking in a different position other then the parking center, or even different positions on the same parking depending on the airplane type, GSX won't use those alternate positions. Docking systems with custom stopping distance works only on FSDT sceneries, because they use docking systems made with GSX.
If you really want to evaluate GSX, you should try on an FSDT scenery (if you have one, GSX is free to use there, since it works in full on FSDT airports, even if it's not purchased), or on one of the Trial airports so, you won't be mislead by possible errors in the AFCAD.
