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.
2004 GMC 2500HD Reg Cab BOSS 9.2 V-Plow
2013 eXmark LazerZ X-Series 60" 34hp EFI (RED) Kohler
2006 Hustler SuperZ 28EFI 60" Power Chute Design OCDC SOLD!
1996 48" SCAG Belt WB 14hp Kawasaki Classic Deck
Echo 2 strokes PB-755
Commercial Lawnboy trim mower
Solo since 1980