I tried a search using ChessBase for players over 2400, and then sorted the result by length, but there were too many games because you cannot filter out blitz and rapid. The overwhelming majority of the games were blitz. I sent them a suggestion regarding adding a title filter and changing the filter for time control.
Looking down the sorted list at the non-blitz games, the games I did look at manually, despite many being almost 200 moves long, continued to have pawn moves until they drew for other reasons. They were not just moving and moving with no progress.
That said, it will be hard to prove either way unless someone happened to witness it, but I tend to doubt it. If a GM game goes that long, it is because one side is pressing, and the other side is probably experienced enough to be looking for the 50th move, and well-aware of what move number is the drawing move number at least roughly. If the player were that much weaker that he does not know the rules (at least the 50-move rule), it is doubtful he would be holding a GM for 75 moves anyway.
Not a perfect answer, but if it has not happened, it is hard to prove a negative.