I looked again....in the picture with the discharge side pointing towards the camera, on the Gravely deck is VERY clear that it doesn't have the rake that the Hustler deck does.
I'm looking at the top plate of the decks across the top of the discharge opening.
1/8" is not enough. 1/4" to 3/8" is where you should probably be.
Also a factor is the rear flap striper. That could be combing the grass up and giving you the stringers. Try removing it.
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