Will & Sara
Good luck on parts or service on one of these babies. A number of years ago I found one source for both, a Lewmar distributor on LI, I believe the following:
Oyster Bay Boat Shop59 Sea Cliff Avenue
Glen Cove, New York 11542
Tel: +1 800-645-7446Fax: +1 800-624-9989
They had parts then and can help you figure out what gypsy you have now, if that is not clear. Perhaps they can source the alternate
gypsy. The manual has reference to an optional 5/8" gypsy: http://en.lewmar.com/support/PDF/S-L...er-D1031-1.pdf
I have found the Seatiger to handlethe 3/8" BBB just fine, its quite a work horse, no particular sweat. However, another concern for weight is the ability of the boat to carry several hundred feet of chain in the bow. Most boats are not designed for such load and will end up bow down. (The ability to sit on her lines with with 200-300 feet of chain, plus two anchors in the bow is one test that identifies a boat DESIGNED to go offshore, and few are.) You might check with other Gulfstar owners on this.
Be that as it may, I would think the 3/8" chain more than adequate for your boat, perhaps you should just consider the 3/8" HT, which fits the BBB gypsy.
Bye the bye, the current issue of Cruising World has a great article about anchor gear and surviving a long gale on the hook(s) and would be good background for your considerations.