If your deck suddenly loses ~50% of cutting capacity, and it's not a belt, then it's likely a broken spring.
The Gravely part has superseded to: 08353300
The instructions in the Operator's Manual are pretty good, but:
1. Yes, you have to remove the mower attachment from the Gravely and then split the drive unit from the deck.
2. After splitting the two mower components, "gently" raise slightly the drive unit, and turn it clockwise 90o. You can now access the spring attachment points.
3. The belt pops off the Rotor Pulley with a screwdriver, and is re-attached by simply inserting it into the sheave slightly and rotating the Rotor Pulley. Easy-peasy.
4. The spring puts up some fight, but with the belt off the Rotor Pulley it's just some fiddling around with the two attachment points. Use the existing spring as your guide on location for installing the new spring.
5. Flip the deck vertical, to ensure that the 3in carriage bolts absolutely seat into the deck. You need a 3/4 and a 9/16 socket for the two carriage bolts and the three nuts.
6. When vertical, now is a good time to clean the deck of old clippings- for guys with the older style of deck without the side discharge, maybe a good time to mortise/grind a discharge port into the deck?
I thought about installing an eye bolt to make re-attachment of a broken spring easier in future. Then I thought, "I'm not smarter than the Gravely Engineering Department" and left well enough alone. KISS
Old and new style decks- the discharge port makes for easier cutting IMO: