I had a 91 Gulfstream. The seats were made by Wise for GW. WD562's are basically the same seat, just a generic version.
Check out this link and see if yours look similar -
http://www.wiseseats.com/wise_marine_offshore_562.html
Some differences;
The generic WD562 is white, the Grady version is beige.
The back side of the upper cushion, (seat back) has a mesh material, as oposed to all vinyl on the Grady version.
The snap pattern on the upper cushion is slightly different, you have to relocate 2 of the snaps, (not a big deal).
If you don't mind the color contrast, you can order replacement cushions for the generic WD562 directly from Wise. Contact info-
http://www.wiseseats.com/wise_marine_contactus.html
Wise told me, they will not sell genuine Grady replacement items directly to the public. You have to go through a Grady dealer.. and you know what that means $$$. I don't recall what I paid for the cushions, (I ordered 2 sets) but imo, it was reasonable.