I'd say that Gravely def needs to do something with thaT belt if it indeed runs under the deck like that.
I think the plastic wheels will hold up for quite a long time if made from good plastic(s). My old Jacobsen 19" trim and rough cut pusher is 42 years old and it has ran on the original plastic wheels for all but 5 or so years of that time. I swapped them out for steel wheels with bearings to make it even esier to push, but the originals are still in good shape. That old mower has covered more ground (and a lot of really rough ground at that) than probably about any mower on this site anywhere. Still going strong too.