As far as I know, the regs say 3 miles off. Dumping at sea is ok up here.
You can go a few routes, get a sanipotti model with pump out fitting, and either plumb it to a macerator, then the bottom of the hull via a 3/4 inch thru hull with ball valve, or to the walk around deck where you go to a pump suck out facility, or both via a Y valve.
The Sannipotti requires no holding tank, it has it own waste tank, as well as it's own FW tank. It's like a potta potti, that stays put, except for the upper half you take out to refill with fresh water. Requires no power to work (except macerator to dump it)
The sannipotti is cheaper than everythig else needed, by about half.
Get the larger holding tank model if you have room (it is not to tall where to be located).
More expensive choices are marine heads (do require holding tank) and vacuflush types, which requires an additional vacuum generator besides the holding tank, this is the most expensive and space hungry choice.
Go to Grady White website, and find the sales catalog for your year boat, and check options list of the Gulfstream, if a marine head is optional then there is room for a holding tank, typically a 9 - 10 gal poly tank. Hoses and Y valve will be 1.5 inch, the output from macertaor is 3/4 in hose.
