All GT games prior to GT5 had that particular area of entrance to the tunnel (or exit in reverse) non-paved - it was a small hill covered with grass.
Without going into debate about "cheating/not cheating", the ordinary *rules* of racing says how track is everything inside the track bounds. In GT5 - as in real life - majority of the bounds are drawn with the white line parallel with the track. On that particular area white lines do not extend to the paved area, so basically that area is more "safety area" than boundaries of the actual track.
From practical point of view, in my community league races we for instance strictly forbid the cutting of that particular area.
Also, you can check the AI routines in B-Spec races where they clearly do not take that run-off portion as in-course path and they strictly follow the boundaries inside the white line.
I presume game does not penalise it because the area is too small and configuration would just produce many problems for the penalty system. But since B-Spec drivers rigourously avoid cutting, we can pretty much say how from the *legality* perspective game "says" it is not a part of the actual track.