"Worn out" is a relative term. Gravely (which was bought by Ariens in 1982) stopped making four wheel tractors nearly 20 years ago, so it isn't surprising that new decks are no longer available. However, you can probably still buy just about every part for a deck from the parts department and assembly one yourself--if you want to spend several thousand dollars. Just about any 40" or 50" deck for a Gravely four wheel tractor will fit you mower, but used decks will be 20 to 50 years old and exhibit varying amounts of wear and tear. The decks use rather common bearing and seals, so a handy person can fix a lot of problems for a modest amount of money.
An earlier Gravely post suggested that some folks are still finding pristine condition tractors with decks for $300 while others are turning down offers for dozen times that amount. You might have some luck with Craigslist, Facebook Market. It seems that Various Facebook groups have taken over most of the posting activity from the other long time Gravely forums and clubs.
It seems that snowblowers for four wheel tractors still have some demand in certain areas of the country. The setup for a front mounted PTO seems to be getting rare and more valuable to some people. You might find someone who would be willing to trade a decent deck for your blower and front PTO setup.