The standard Gravely 'L' had a max speed of about 4 mph in high and 3 mph in low. I think the 'LI' has a max speed of 3 mph in high and 2.25 mph in low. Other than the gearing difference, they are the same.
The PTO is not reversible, but it does have a "high" and "low" speed controlled by the high-low lever that controls ground speed. The 'dog gear' on the PTO shaft never disengages, it is supposed to slide in and out on its shaft. When it slides out on its shaft, it engages the attachment clutch and drives the implement.
Does this answer your questions?